9-36th grade math

Finding a Percent of a Number

Objective

• To use a proportion to set up percent problems and solve using cross products.

• Why? To know find discounts and sales prices.

California State Standards NS 1.4 : Calculate given percentages of

quantities …AF 2.0: Use … rules to solve problems involving …

proportions.MR 2.2: Apply strategies and results from simpler

problems to more complex problems.MR 3.2: Note the method of deriving the solution

and demonstrate a conceptual understanding of the derivation by solving problems.

Percent Problems: 3 ways

• Find a part, given a percent and the whole– “What is 40% of 75?”

• X = 40% x 75

• Find a percent, given the whole and a part– “What percent of 50 is 20?”

• X% x 50 = 20

• Find a whole, given a percent and a part– “35% of what number is 70?”

• 35% x X = 70

How to Find a Percent of a Number

1) Write equation, using the % symbol.

a) ‘of’ means xb) ‘is’ means equals

2) Translate the equation: change the % to a decimal and solve.

3) Check answer for reasonableness

How much money will you save if you buy jeans on sale that originally costs $60. Sale is 30% off.

60 x 30%= 60 x .30= 18.00You will save $18

Writing a Proportion to Find a Percent of a Number

How to write a proportion1) Write the first ratio: The

% will change into a fraction with 100 as the denominator

2) Write the second ratio: The $ amount will be the

denominator and n will be the numerator.

3) Solve using cross products

Find 40% of 75.40 = n100 75

100n = 3000n = 3000 ÷ 100n = 30

Try It!

1) 40% of 120

2) 25% of 245

3) 120% of 60

4) 6% of 25

1) .40 x 120= 48

2) .25 x 245= 61.25

3) 1.20 x 60 =72

4) .06 x 25= 1.5

Try Some More!

5) 10% of 46

6) 35% of 90

7) 45% of 50

8) 100% of 7

5) .10 x 46 = 4.8

6) .35 x 90 = 31.5

7) .45 x 50 = 22.5

8) 1.00 x 7 = 7

Objective Review

• To use a proportion to set up percent problems and solve using cross products.

• Why? You now know to find discounts and sales prices.

• Proportional reasoning can be used to set up problems involving finding a percent of a number.

• The proportion can be solved using cross-products.

Independent Practice

• Complete problems 10- 17

• Copy original problem first.

• Show all work!

• If time, complete Mixed Review: 18-27

• If still more time, work on Accelerated Math.