For me 7 and 8 were always difficult to work with since school. If it is 9 for me it is 10-1, if it is 6 for me it is 5+1. Even now as a grown-up i tend to avoid these numbers in addition and multiplication.

I would probably do

20 + 40 8 + 7 and finally 60 + 15

But even in that way for me there is much to remember: 1st sum, 2nd sum and then to sum two sums. You wouldn't believe me but every time I have to ADD 8 and 7, I simply can't remember THAT sum. With the other sums I have no problem.