Have a look at the examples on reaction maths called subtract double.
www.mathsisfun.com/games/reaction-math.htmlIf your son is repeatedly getting 100% on these maths fact tests, then perhaps it's time to raise the ceiling. Can you supply the teacher with an alternative 'test'?
thanks for the great tip! That site looks wonderful! It looks like we could print out lots of variations of double digits and then I could time him. They even give the answers. I'm going to give it a try and see if he would like to do this instead of his normal timed tests.