Compatible Numbers

Compatible numbers are numbers that help make doing mental math more manageable. Compatible numbers are used to get quick and rough approximations, not precise answers. Consider finding the estimation of the following quotient. 612 ÷ 9 It might seem difficult to divide these numbers using mental math. However, both numbers can be rounded to compatible numbers — 612 to 600 and 9 to 10. An estimate for 612 ÷ 9 can then be found mentally by dividing 600 by 10, which is equal to 60. That means the given quotient is about 60.

The compatible numbers we chose are not the only options. Any numbers that are easily divisible and close to the original numbers are worth exploring. For example, rather than rounding 612 to 600, let's try 630. Since 63 is a multiple of 9, 630 is divisible by 9. The estimate for the quotient is then 630÷ 9 = 70.