Estimate the difference 8 7/12 - 4 7/8 - 4/10

When you are told to estimate, you should be able to do the problem in your head.
8 7/12 is close to 8 1/2
4 7/8 is very nearly 5
4/10 is very nearly 1/2

8 1/2 - 5 - 1/2 First of all you can take the 1/2 away from 8 1/2
8 1/2 - 1/2 = 8
Now take away the 5

8 - 5 = 3

So the answer is very nearly
3 <<<<< answer.

Just for fun, I'll get the exact answer. Don't hand this one  in. It is 3 37/120
You learn to estimate because you are trying to guess whether or not you've entered everything correctly into your calculator. When writing a test, this is a very handy skill to have developed.