After rounding numbers to the tens place adding the new numbers together is much easier. The ones place is filled with zeros so all that really has to be added together are the digits in the tens place.
23 + 38 = ?
Ex. 23 becomes 20 and 38 becomes 40
20 + 40 = ___ Since both ones place have a zero, the answer will have a 0 in the ones place. So all that is left is to add the 2 and 4 which equal 6. Since the 2 and 4 were in the tens place, the number 6 will fill the tens place in the new answer.
20 + 40 = 60 and 23 + 38 = 61
The rounded values give a very close answer to the actual value so this is a good estimate.
25 + 45 = ? Rounding: 30 + 50 = 80
Actual value: 25 + 45 = 70
So the answer 80 would not be a good estimate because it is very far from the actual value.