Concept

To use mental math to add or subtract two numbers, first round one of the numbers up or down to the nearest five or ten. If rounding up, subtract the amount that was rounded from the equation. If rounding down, add the amount that was rounded to the equation.

Rules

a  -  b  =  c
Round one of the numbers up or down to the nearest five or ten.

a (+/-n) - b = c

If rounding up, subtract the amount that was rounded from the equation.
a - b (- n) = c

If rounding down, add the amount that was rounded to the equation.
a - b (+n) = c

Then solve for c.

Example

How would you break the following expression to solve easily?

Drag and drop the correct numbers in the blank spaces to complete the expression.

198 - 73

Solution

200 - 73 - 2

The easiest way to solve this equation is to round 198 up to 200 by adding 2. Since 198 was rounded up by 2, 2 must be subtracted from the equation.

Prerequisite Skills

Add Decimals (4.16.6)

Subtract Decimals (4.16.7)