Study this problem:
2000 - 555 --------
If we are unsure of our answer we can check our answer to see if it is reasonable. Since 2000 - 555 is going to have an answer close to the answer for 2000 - 500, we can feel comfortable if our answer is close to it. Since 2000 - 500 equals 1500 and the answer we got for 2000 - 555 is 1445, we can feel that we at least computed a reasonable answer.
On the next page are some problems for you to try.