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)