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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.