What are compatible numbers?

They are numbers that "work well" or are friendly with each other.

In a problem like 16 + 4+8 + 12 + 5, you could say 16 + 4 is 20 and 8+ 12 is 20. 20 + 20 is 40 and with the other five you would just add it so it would be:

16+4=20

8+12=20

20+20=40

40+5=45

So they are numbers that most people would find it easy to add, subtract, or divide.

232 ÷ 11 may be hard to do in your head.

We can use the compatible number 12 and 240 since that division is easy to

do in your head. 12 goes into 240 20 times. The easy way to do this in your head is to note that 12 goes into 120 10 times so it goes into 240 twenty times.

This gives us an estimate of the answer when we consider that both number were increased by 1.

Say you wanted to multiply 288 by 415 . We note that the compatible numbers for 288 and 415 are 300 and 400

In the addition example, we group certain numbers first and that makes addition easy.