How to estimate the product of multiplication

Students estimate a product by rounding each factor to an easy number, then multiplying.

• Round both factors to the nearest ten: 56 × 24 becomes 60 × 20 = 1,200
• Round to different places: 431 to the nearest hundred, 71 to the nearest ten
• Multiply numbers ending in zeros: 600 × 7,000 = 4,200,000
• Say whether a factor was rounded up or down

Rounding to any place is 4.NBT.A.3; estimating a product is a quick check on the 5.NBT.B.5 algorithm.

Print page one and follow the started example: two pink boxes for rounding, one box for the multiplication, one for the estimate.

• Look at the digit to the right of the rounding place: 5 or more rounds up.
• Multiply the nonzero digits, then attach all the zeros: 4 × 7 = 28, so 400 × 70 = 28,000.
• Write the estimate in the rounded box on the right.

Page two shows every rounding and estimate.

• Rounding the wrong way: 24 rounds down to 20, not up to 30.
• Counting zeros badly: 600 × 7,000 has five zeros, so the estimate is 4,200,000.
• Calling the estimate exact: 34 × 79 is really 2,686, close to the estimate of 2,400.

Note: the key says "71 is rounded up to 70"; it is rounded down. Next, compare each estimate with the exact product.