1. Pre-Algebra 8-5 Estimating with Percents Learn to estimate with percents.

2. Pre-Algebra 8-5 Estimating with Percents Some problems require only an estimate. Estimates involving percents and fractions can be found by using friendly numbers, numbers that go well together because they are easy to work with. 12 24  friendly 13 24 not friendly

3. Pre-Algebra 8-5 Estimating with Percents Estimate. A. 21% of 66 Example 1A: Estimating with Percents Instead of computing the exact answer of 21%  66, estimate. 21% =  21 100 20 100 Use friendly numbers, 20 and 100.  1 5 Simplify. 66  65 Use friendly numbers, 65 and 5. 1 5  65 = 13 Use mental math: 65 ÷ 5. So 21% of 66 is about 13.

4. Pre-Algebra 8-5 Estimating with Percents Estimate. B. 36% of 120 Example 1B: Estimating with Percents Instead of computing the exact answer of 36%  120, estimate. 36%  35% Round.  30% + 5% Break down the percent into smaller parts.  3  10% + 5%

5. Pre-Algebra 8-5 Estimating with Percents Example 1B Continued = 36 + 610% of 120 is 12, so 5% of 120 is 6. = 42 So 36% of 120 is about 42.

6. Pre-Algebra 8-5 Estimating with Percents The diameter of the Moon is about 2160 miles. If the diameter of the Moon is about 27% of the diameter of Earth, what is the approximate diameter of Earth? Example 2: Problem Solving Application

7. Pre-Algebra 8-5 Estimating with Percents Example 2 Continued 1 Understand the Problem The answer is the approximate diameter of Earth. List the important information: The diameter of the Moon is about 2160 miles. The diameter of the Moon is about 27% of the diameter of the Earth. Let e represent the diameter of the Earth. 2160  27%  e Diameter of the Moon  27%  Diameter of Earth

8. Pre-Algebra 8-5 Estimating with Percents Use compatible numbers: 2160 is close to 2000; 27% is close to 25% 2 Make a Plan Think: The numbers 2160 and 27% are difficult to work with. Example 2 Continued Find an equivalent ratio for 25%. 25% = = 25 100 1 4

9. Pre-Algebra 8-5 Estimating with Percents Solve 3 Think: 2000 is of what number? 1 4 2000 =  e 8000 = e The diameter of Earth is approximately 8000 miles. Example 2 Continued 1 4

10. Pre-Algebra 8-5 Estimating with Percents Solve 3 Think: 2000 is of what number? 1 4 4  2000 = e 8000 = e The diameter of Earth is approximately 8000 miles. Example 2 Continued

11. Pre-Algebra 8-5 Estimating with Percents Look Back4 25% of 8000 mi is, or 2000 mi. This is the approximate diameter of the moon. 8000 4 Example 2 Continued

12. Pre-Algebra 8-5 Estimating with Percents Estimate. A. 29% of 86 Try This: Example 1A Instead of computing the exact answer of 29%  86, estimate. 29% =  29 100 30 100 Use friendly numbers, 30 and 100.  3 10 Simplify. 86  90 Use friendly numbers, 90 and 10. 3 10  90 = 27 Use mental math: 90 ÷ 10. So 29% of 86 is about 27.

13. Pre-Algebra 8-5 Estimating with Percents Estimate. B. 44% of 130 Instead of computing the exact answer of 44%  130, estimate. 44%  45% Round.  40% + 5% Break down the percent into smaller parts.  4  10% + 5% Try This: Example 1B

14. Pre-Algebra 8-5 Estimating with Percents = 52 + 6.5 10% of 130 is 13, so 5% of 130 is 6.5. = 58.5 So 44% of 130 is about 58.5. Try This: Example 1B Continued