65+23 ... Use rounding or compatible numbers to estimate the sum

What is the answer?

Well, if you want to estimate the sum, then we must round the two numbers we are adding.

65 can be rounded to 70, since 5 is ≥ 5.

23 can be rounded to 20, since 3 < 5.

70 + 20 = 90

We can estimate the sum to be 90.

(If you want the actual sum it would be 65 + 23 = 88)