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Estimating with Whole Numbers
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Vocabulary estimate ­ to find an answer close to the exact answer by rounding or other methods
compatible numbers ­ numbers that are close to the given number that make estimation or mental calculation easier
underestimate ­ an estimate that is less than the exact answer
overestimate ­ an estimate that is greater than the exact answer
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Example 1 Estimating a Sum or Difference by Rounding Estimate the sum or difference by rounding to the place value indicated. A. 5,439 + 7,516; thousands Round 5,439 _______________. Round 7,516 _______________. The sum is about ________________. 4
B. 62,167 ­ 47,511; ten thousands
Round 62,167 ________________.
Round 47,511 ________________. The difference is about ____________________. 5
Example 2 Estimating a Product by Rounding Mr. Escobar is planning a graduation celebration for the entire eighth grade. There are 9 eighth­grade homeroom classes of 27 students. Estimate how many cups Mr. Escobar needs to buy for the students if they all attend the celebration. 6
Example 3 Estimating a Quotient Using Compatible Numbers 7
Think and Discuss 1. Suppose you are buying items for a party and you have $50. Would it be better to overestimate or underestimate the cost of the items? 2. Suppose your car can travel between 20 and 25 miles on a gallon of gas. You want to go on a 100­mile trip. Would it be better to overestimate or underestimate the number of miles per gallon your car can travel? 8
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