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 + 24, take 1 from 24 to get 23 and add the 1 to the 329 to get 330, so you end up with 330 + 23 = 353.
Thid works for many numbers. Say you adding 329 + 24, take 1 from 24 to get 23 and add the 1 to the 329 to get 330, so you end up with 330 + 23 = 353.