Here is a little twist on the usual adding three numbers type of worksheet. On this page the third number is missing but the answer is given. What makes this harder is that at least two mental calculations have to be made to reach a correct answer.
Probably the easiest way to do this is to add the first two numbers together and take that total away from the final answer.
Another way of doing this is to count back each of the two numbers in turn and a n umber line is provided for this method.