number bonds

[NAB3]

"instant recall of number bonds up to twenty"
This is what my son's target is for the holidays. What does it mean? It wasn't called number bonds when I went to school. Whatever it is!

So, do I ask him 1=? is 20, 2+? equals 20?, etc.

1+?=20

It's a good game to play in the car--you just call out the number and they tell you what it needs to make it up to 20.

Great idea. Thanks. Only trouble will be my dd who is 4 next week will probably be able to answer quicker than my 6 year old! Cue shouting.....