I would try with dienes block so that they can see that if they are adding ten, then the unit blocks stay the same, if they had 100 then the tens and unit blocks remain the same. I also do lots of target practise on calculators. Ask them to put in a number such as 149 on the calculator. You then tell them that one of the digits is a target and to hit it they must put in a calculation that turns that digit only into a 0. So if the target is the 4 they could either input -40 or +60.