There is an explanation in my assignments that say a good and easy way of grouping is to find a way to make 100, then add or subtract whatever is next. In this case (70+30) makes 100, then you must add 72, and because its a double diget (i cant spell, sorry!) and and so are the two zeros in the triple diget, so it should not be hard to put the two numbers together if there are zeros, (plural). I hope you understand this!
