Estimating Percent Mentally

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Estimating Percent Mentally Estimating Percent Mentally 10% 10% Lola borrowed $125 from a pawn shop. She’ll have to pay back the loan, plus the pawn shop charges an additional 9.9% fee. ≈ how much will her additional fee be? Step 1: Step 1: Look at your cost and your percent. Do they need rounding? ~ ~ $125 is OK, but let’s round 9.9% to 10% 10% 10% Step 2: To find 10%, slide the decimal 1 space to the left 1 space to the left. 5% 5% do the 10% trick cut that number in half 15% 15% do the 10% trick cut that number in half add both 20% 20% do the 10% trick double that number 25% 25% do 20% + 5% or mentall y divide 50% 50% round your number cut it in half 0 0 Fee is Fee is about $12 about $12 or $13. or $13.

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0. 10%. Step 1: Look at your cost and your percent. Do they need rounding?. Estimating Percent Mentally. ~ $125 is OK, but let’s round 9.9% to 10%. Fee is about $12 or $13. Step 2: To find 10%, slide the decimal 1 space to the left. 5%. 15%. 20%. 25%. 50%. - PowerPoint PPT Presentation

Transcript of Estimating Percent Mentally

Page 1: Estimating Percent Mentally

Estimating Percent MentallyEstimating Percent Mentally

10%10% Lola borrowed $125 from a pawn shop. She’ll have to pay

back the loan, plus the pawn shop charges an additional

9.9% fee. ≈ how much will her additional fee be?Step 1: Step 1: Look at your cost and your percent. Do they need rounding?

~ ~ $125 is OK, but let’s round 9.9% to 10%

10%10%

Step 2: To find 10%, slide the decimal 1 space to the left1 space to the left.

5%5%∞ do the

10% trick

∞ cut that number in half

15%15%∞ do the

10% trick

∞ cut that number in half

∞ add both numbers

20%20%∞ do the

10% trick

∞ double that number

25%25%∞ do 20%

+

5%

or

∞ mentally divide the number by 4

50%50%∞ round

your number

∞ cut it in half

00

Fee is Fee is about $12 about $12

or $13.or $13.

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Estimating Percent MentallyEstimating Percent Mentally

about $0.05 - $0.06 (5 or 6 cents)

≈$0.40 - $0.41 (about 40 or 41 cents)

≈ $7.00 - $7.40

≈ $59 - $60

≈ $1000 - $1200

≈ $1,6 00,000,0 00 - $2,000,000,000 (1.6 trillion to 2 trillion)

≈ $1 - $1.20 (10%)

≈ $800 (10%)

≈ $60 + $30 = $90 (10%) + (5%) = (15%)

= $.02 (2 cents) (25%)

≈ $16 4

≈ $50,000 - $55,000 (50%)

-->$0.08 = (10%) 2

≈$.08 (or 8 cents)

--> $1 2

≈$0.08 (8 cents) 4

≈$28,000 4

≈ $7,000 (25%)

≈$100,000 2≈ $1,000,000 2

$9 000 00 or $900,000

$.04 (or 4 cents) (5%)

= $.50 - $.60 (5%)

≈ $40000 + $20000 = $60,000 (10%) + (5%) = (15%)

≈ $.1 2 (10%)

+ $.06+ (5%)

= $.15 - $.20= (15%)

≈ $4 (10%) •

● (2)● (2)

= $8= (20%)

≈ $50 00 (10%)

● (2)● (2)

= $10,000 (20%)

≈ $2 400 00 ● 2 = $40,000 - $50,000 (10%) ● 2 = (20%)

≈ $4 - $5 (25%)

≈ $500,000 - $550,000 (50%)

≈ $190 - $200 (50%)

≈$400 2

--> $800 2

= $400 (5%)

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Tina bought a set of golf balls sells for $20 and the sales taxsales tax is 5.75% of the price. What is the total

cost of the set?To find the sales tax, ...

Next, add the sales tax to the regular price.

Answer: The Answer: The total cost total cost of the set of golf balls is of the set of golf balls is $21.15$21.15

Sales TaxSales Tax

(20)

(price)(tax rate) as a decimal

1.15 (sales tax)

+ 20.00 (reg price)

$21.15 (total cost)

MultiplyMultiply the price and the tax rate (as a decimal)

(.0575)

5.75%5.75%

.0575.0575

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Linda and Bob went to dinner, received lousy service, and decided to tiptip the server only 12%. If the prepared foods taxtax is 4.75%, and they ordered $68.19 of food and

drinks, what will they pay?To find the tip, ...

Finally, add them up.

AnswerAnswer: Linda : Linda

and and Bob Bob will will pay pay

$79.61$79.61..

Tip, then Tax

(68.19)

8.18

(68.19)3.24

(price) (tip rate) as a decimal

Next, to find the tax

68.19 + 8.18 + 3.24

MultiplyMultiply the price and thetip rate (as a decimal).

(price) (tax rate)

Multiply Multiply the price and the tax rate.

(.12) 12%12% .12.12

4.75%4.75%

.0475.0475

(.0475)

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Mort and Teddy receive a “15% off” coupon for Bob’s Burgers. They order $22.57 of food and drinks, decide to tip 20%, and

the food tax is 5 %. What will they pay in total?

To find the tip, multiply the new price new price and the tip rate (as a decimal)

Finally, add them up.

They paid They paid $24.07$24.07

Discount, then Tip, then Tax

(19.1819.18)(.20)

= 3.84

(19.1819.18)(.055)

= 1.05

19.1819.18 + 3.84 + 1.05

2

1

To find the discount, ...

(22.57)(.15)

3.39

Multiply the price and the discount rate (as a decimal)

22.57 - 3.39 = = 19.1819.18

(price) (discount rate) as a decimal

Now, subtract the discount from the old price to get the new pricenew price.

To find the tax, multiply the new price new price and the tax rate (as a decimal)

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DECIMAL…move the decimal 2 spaces to the

right.

Converting Between Decimal and Percents

PERCENT

From a to a 0.13

= .13 13%

=

0.9 = .90=90%

0.0027

= 0.27

=0.27%

0013 = 1300 1300%

• =

DECIMAL…move the decimal 2 spaces to the

left.

PERCENT

From a to a 0.78

=78 78%=•

0.05

= 5 5%=• 1.75

= 175

175%

= 0.0008%

= 00.000008

•0

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To change a fraction to a percent,1. Check the denominator.

7 20

=• 5 100• 5 35=35%35%

216 300

= ÷ 3 100 ÷ 3 72 = 72%72%

3 7

* Can it be changed to 100?

2. Rewrite the fraction with a denominator of 100.

What if the denominator can’tcan’t be changed to 100?

a. Divide

=3 ÷ 7

0.4285…

b. Change to a percent * slide the decimal 2 to the

right

42.85

d. Round to the nearest tenth.

42.9% c. Slap a percent sign on it.

%

yes

3. Write it as a percent.

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To change a percent to a fraction,

1.Re-write the percent as a fraction over 100

8 100

=

2. Simplify.

8%8%

275100

÷ 4 ÷ 4

2 25

275%275% ÷ 25 ÷ 25

Converting Percents to Fractions

11 4

==

3.6100

3.6%3.6% ÷ 4 ÷ 4

9_250

== 36_ 1000

==*If there’s a decimal, move it to the end. … but, what you do to the numerator.. … you also do to the denominator

•0

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Modeling Fractions, Decimals & Percents.

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Gene found a bank offering a certificate of deposit that pays 4% simple interest. He has $1,500 to invest. How much interest will he earn in 3 years?To find interest, use a formula

Replace the variables:~ P with $1,500~ R with .04~ T with 3.

Multiply.Answer: Gene will earn

$180 in interest in 3 years.

Finding Interest

INTERESTINTERESTextra money

someone pays when

they borrow money.

I = P•R•T

I = 1500 • .04 • 3

I = 180

PRINCIPALPRINCIPALamount of money you borrow or

lend.

RATERATEinterest

rate written written

as a as a decimaldecimal

TIMETIMEyears of years of the loanthe loan

Why .04? All % must be written

as decimals.

$

I = P • R • T

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Joan works for West Bank, and she just made a car loan at 8.9% interest rate8.9% interest rate for 5 years5 years. The bank will earn $5,340$5,340 in interestinterest. How much principalprincipal did she loan? 1. Replace the variables:

~ I with 5340. ~ P with p ~ R with .089 ~ T with 5.

Finding Principal, Rate, or Time

5340= p • .089 • 5

I = P • R • T

5340= p • .445------- -------.445 .445

12,000 = p Joan loaned $12,000 in principal.

2. Multiply (rate • time)

3. Divide to find p.

Collette put $400 in principal in a savings account for 2 years, and she earned $7.20 in interest. What interest rate What interest rate was she earning?

Ralph just finished paying off a $3500 loan. He paid an additional $1741.25 in interest on his 19.9% loan. What was the length of time on this loan?

I = P • R • T

7.2 = 400 • R • 27.2 = 800 • R----- ------ 800 800

0.009 = R

The The rate is rate is 0.009, 0.009,

or 0.9%or 0.9%

1741.25 = 699.5 • T------------ ---------- 699.5 699.5

2.5 = T

1741.25 = 3500 • 0.199 • T

It was a 30 month It was a 30 month loan.loan.

I = P • R • T

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