Which term refers to numbers that are easy to compute mentally?

• A. Compatible numbers
• B. Prime number
• C. Rate
• D. Proportion

Answer: (A)

Compatible numbers are numbers chosen because they make mental math smoother. The idea is to pick pairs or groups of numbers that work well together so you can get a result quickly and with fewer steps. This often means using numbers near multiples of ten or numbers that combine to round, easy totals.

For example, multiplying 8 by 25 is simple in your head because 25 pairs with 4 to make 100. So you can think of 8 × 25 as (2 × 4) × 25 = 2 × 100 = 200. Another handy case is using a multiple of ten alongside a factor: 60 ÷ 5 is easy since 60 is divisible by 5 and the quotient is 12. These kinds of pairings are the essence of working with compatible numbers.

Prime numbers, rates, and proportions describe different ideas—prime numbers are about divisibility, while rates and proportions describe relationships between quantities rather than ease of mental calculation.