January 21, 2010Math 201 OBJECTIVE: Students will be able to estimate a a percent of a quantity AIM: How can we use rounding and mental math to help us estimate? STANDARDS: 6.N.26 PRIOR KNOWLEDGE: What is 52% of 225?

Do the math: Vocabulary to know for this lesson: approximate – to give an answer that is nearly exact rounding a number – to replace a number with another number with fewer digits that is approximately the same value mental math – to use easy numbers like ones and tens in our minds to approximate

MINI LESSON: You can use rounding and mental math to determine an approximate percent of a number Example What is 41% of 289? Step 1Round 41% to the nearest 10% 41% rounds down to 40% Step 2Round 289 to the nearest hundred 289 rounds up to 300 Step 3Use mental math to find 10% of % of 300 is 30

Step 3Find 40% of % of 300 is 4 times 10% of X 30 = 120 Solution:41% of 289 is about 120 Example 2 (student response) Isabel received a “15% off” discount coupon from Staples. If she spent $23.85, about how much of a discount did she receive? Step 1Round $23.85 to the nearest dollar

Step 2Use mental math to find 10% of $24 Step 3Use mental math to find 5% of $24 (5% of $24 is ½ of 10% of $24) Step 4Add 5% of $24 to 10% of $24 (because 5% + 10% = 15%) Solution:Isabel received a discount of about ________________ from Staples.

About how much is 32% of $500? Let’s check it out. Round 32% to the nearest 10% __________________ Find 10% of $500 _____________________ How many more times is 30% of $500 than 10% of $500? _____________ Multiply 3 by 10% of 500 _____________________ So 32% of 500 is about _____________________

Homework 1)Estimate 72% of 400 2)Estimate 78% of 899 3)Sales tax on a Lady Gaga CD is 9%. About how much sales tax is added if the CD costs $18.99?