Compatible numbers are pairs of numbers that are easy to add, subtract, multiply, or divide mentally.

When using estimation to approximate a calculation, replace actual numbers with compatible numbers.

500 + 300 = 800

The numbers 500 and 300 are compatible for addition, since the sum of 800 can be easily calculated mentally.

513 + 299 = 812

The numbers 513 and 299 are not compatible for addition, since the sum (812) cannot be easily calculated mentally. To estimate 513 + 299, replace 513 and 299 with the compatible numbers 500 and 300. An estimate of 513 + 299 is found by mentally calculating 500 + 300 = 800.

19.4 − 3.8 = 15.6

The numbers 19.4 and 3.8 are not compatible for subtraction, since the difference (15.6) cannot be easily calculated mentally. To estimate 19.4 − 3.8, replace 19.4 and 3.8 with the compatible numbers 19 and 4. An estimate of 19.4 − 3.8 is found by mentally calculating 19 − 4 = 15.

19.4 × 3.8 = 73.72

The numbers 19.4 and 3.8 are not compatible for multiplication since the product (73.72) can't be easily calculated mentally. To estimate 19.4 × 3.8, replace 19.4 and 3.8 with the compatible numbers 20 and 4. An estimate of 19.4 × 3.8 is found by mentally calculating 20 × 4 = 80.

721 ÷ 70 = 10.3

The numbers 721 and 70 are not compatible for division, since the quotient (10.3) can't be easily calculated mentally. To estimate 721 ÷ 70, replace 721 with 700 so that the numbers involved are compatible. An estimate of 721 ÷ 70 is found by mentally calculating 700 ÷ 70 = 10.