Pre-Algebra 8-3 Finding a Number When the Percent Is Known 8-3 Finding a Number When the Percent Is Known Pre-Algebra Warm Up Warm Up Problem of the Day Problem of the Day Lesson Presentation Lesson Presentation

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Warm Up 1. What percent of 20 is 18? 2. What percent of 400 is 50? 3. 9 is what percent of 27? is what percent of 4? 90% 12.5% Pre-Algebra 8-3 Finding a Number When the Percent Is Known 625% %

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Problem of the Day The original price of a sweater is $40. The sweater goes on sale for 75% of its original price. Later, the sweater goes on clearance for 50% of its sale price. What is the clearance price of the sweater? $15

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Learn to find a number when the percent is known.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known The Pacific giant squid can grow to a weight of 2000 pounds. This is 1250% of the maximum weight of the Pacific giant octopus. When one number is known, and is relationship to another number is given by a percent, the other number can be found.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known 60 = 0.12n 60 is 12% of what number? Additional Example 1: Finding a Number When the Percent Is Known Set up an equation to find the number. 60 = 12%  n = n Divide both sides by = n 60 is 12% of 500. Set up an equation % =

Pre-Algebra 8-3 Finding a Number When the Percent Is Known 75 = 0.25n 75 is 25% of what number? Try This: Example 1 Set up an equation to find the number. 75 = 25%  n = n Divide both sides by = n 75 is 25% of 300. Set up an equation % =

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Anna earned 85% on a test by answering 17 questions correctly. If each question was worth the same amount, how many questions were on the test? Additional Example 2: Application Choose a method: Set up a proportion to find the number. Think: 85 is to 100 as 17 is to what number? = n Set up a proportion.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Additional Example 2 Continued There were 20 questions on the test. 85  n = 100  17 Find the cross products. 85 n = 1700 n = Solve for n n = 20

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Try This: Example 2 Choose a method: Set up a proportion to find the number. Think: 80 is to 100 as 20 is to what number? = n Set up a proportion. Tom earned 80% on a test by answering 20 questions correctly. If each question was worth the same amount, how many questions were on the test?

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Try This: Example 2 Continued There were 25 questions on the test. 80  n = 100  20 Find the cross products. 80 n = 2000 n = Solve for n n = 25

Pre-Algebra 8-3 Finding a Number When the Percent Is Known A. A fisherman caught a lobster that weighed 11.5 lb. This was 70% of the weight of the largest lobster that fisherman had ever caught. What was the weight, to the nearest tenth of a pound, of the largest lobster the fisherman had ever caught? Additional Example 3A: Life Science Application Choose a method: Set up an equation. Think: 11.5 is 70% of what number? 11.5 = 70%  n Set up an equation.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Additional Example 3A Continued 16.4  n The largest lobster the fisherman had ever caught was about 16.4 lb = 0.70  n 70% = 0.70 Solve for n n =

Pre-Algebra 8-3 Finding a Number When the Percent Is Known B. When a giraffe is born, is approximately 55% as tall as it will be as an adult. If a baby giraffe is 5.2 feet tall when it is born, how tall will it be when it is full grown, to the nearest tenth of a foot? Additional Example 3B: Life Science Application Choose a method: Set up a proportion. Think: 55 is to 100 and 5.2 is to what number? = h Set up a proportion.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Additional Example 3B Continued 55h = 520 This giraffe will be approximately 9.5 feet tall when full grown. 55  h = 100  5.2 Find the cross products h = Solve for h. h  9.5

Pre-Algebra 8-3 Finding a Number When the Percent Is Known A. Amy caught a fish that weighed 15.5 lb. This was 85% of the weight of the largest fish that Amy had ever caught. What was the weight, to the nearest tenth of a pound, of the largest fish that Amy had ever caught? Try This: Example 3 Choose a method: Set up an equation. Think: 15.5 is 85% of what number? 15.5 = 85%  n Set up an equation.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Try This: Example 3 Continued 18.2  n The largest fish that Amy had ever caught was about 18.2 lb = 0.85  n 85% = 0.85 Solve for n n =

Pre-Algebra 8-3 Finding a Number When the Percent Is Known B. When Bart was 12, he was approximately 85% of the weight he is now. If Bart was 120 lb, how heavy is he now, to the nearest tenth of a pound? Try This: Example 3 Choose a method: Set up a proportion. Think: 85 is to 100 as 120 is to what number? = w Set up a proportion.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known 85w = Bart weighs approximately lbs. 85  w = 100  120 Find the cross products w = Solve for h. w  Try This: Example 3 Continued

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Three Types of Percent Problems 1. Finding the percent of a number. 15% of 120 = n 2. Finding the percent one number is of another. p% of 120 = Finding a number when the percent is known. 15% of n = 18 You have now seen all three types of percent problems.

Pre-Algebra 8-3 Finding a Number When the Percent Is Known Lesson Quiz is 12 % of what number? is 25% of what number? 3. 44% of what number is 11? % of what number is 145? 5. Larry has 9 novels about the American Revolutionary War. This represents 15% of his total book collection. How many books does Larry have in all?