Compatible numbers are pairs of numbers that are easy to add, subtract, multiply, or divide mentally. They are close in value to the actual numbers, which makes estimating the answer and solving problems easier. Compatible numbers are also called friendly numbers.

Examples of compatible numbers include:

• Addition: Numbers ending with 5 or 0 are always easy to work with. Also, number pairs that add up to a multiple of 10 (or a number ending with 0) make addition easier. For example, 65 + 15 = 80, and 513 + 299 can be estimated as 500 + 300 = 800.

• Subtraction: Compatible numbers for subtraction are those that are close to each other and can be rounded to the nearest ten or hundred. For example, to estimate 19.4 - 3.8, we can replace 19.4 and 3.8 with the compatible numbers 19 and 4, and mentally calculate 19 - 4 = 15.

• Multiplication: Multiples of ten are great compatible numbers in multiplication. For example, to estimate 19.4 x 3.8, we can replace 19.4 and 3.8 with the compatible numbers 20 and 4, and mentally calculate 20 x 4 = 80.

• Division: Compatible numbers for division are those that are close to each other and can be rounded to the nearest ten or hundred. For example, to estimate 721 ÷ 70, we can replace 721 with 700 and mentally calculate 700 ÷ 70 = 10.

Compatible numbers are not always whole numbers, but can also be decimal numbers.