I always had trouble adding numbers past a multiple of ten (so like 66+7) so the way I do it is to figure out how much I need to get to the next one (so 4) and then figure out what I have left and then add it on (3, so the answer is 73). If it's larger numbers like 47 then I'll add the tens and do the same as above once I've only got single digits to worry about. Different minds work in different ways.