Using Two-Digit Subtraction

The most common method of estimating sums and differences is rounding each number to a specific place and then subtracting the rounded numbers. In this chapter, numbers are rounded to the nearest ten or hundred with the help of a number line.

Mental math can often be used to complete estimates. At this grade level, however, explain that errors can be more easily identified if children write down their work when estimating answers.

Estimate: 699	− 277
699	→	700	Round each number to the nearest hundred.
− 277	→	− 300	Subtract the rounded numbers.
		400

Children are also reminded that addition can be used to check subtraction. So, to check the estimate above, children should think 400 + 300 = 700.

Teaching Model 13.5: Use Addition to Check Subtraction