Number form means to write it a a regular number without words, just as if you were putting them in a calculator.  Sometimes it is called standard form.

Usually a compatible number is one that will make easier to add the number in your head.  If your take 51 and add it to 329, take the 1 and give to the 329 to make 330, so then you have 330 + 50 = 380.   Thid works for many numbers.  Say you adding 329 … Read more

I may be mistaken, but actually a compatible number is easier than that.

Say you are adding 3 + 5 + 6 + 7 + 4

If you combine 3 + 7 = 10 and  6 + 4 = 10

you then have 10 + 10 + 5 = 25