For example,
1. A student says 741.

2. Another student says 811.

3. Then you write down 188.

4. A student says 265.

5. Then you write down 734.

6. A student says 656.

7. Then you write down 343.

8. And you immediately draw a line under the seven numbers and write the sum: 3,738!

1. The first student gives 741.     You now know the answer is going to be 3,738.     The sum will be 3,000 + 3 less than the first number given.     The reason is that for the next three numbers that students give you,     you add the difference between 999 and the number given.     Therefore, you are adding in three 999s.     999 is 1000 - 1, so three 999s is 3000 - 3.

2. The second student gives you 811, so you write down 188 (each column must add to nine).

3. The third student gives you 265, so you add in 734.

4. The fourth student gives you 656, so you add in 343.