Question 37867
This can be tricky at first, but it is an important understanding...
If two things add to make 90, and you know one of them, how do you find the other?
Don't you subtract it from 90?
If I tell you one angle is 20, the other is 90 - 20 or 70, right?
If I tell you one angle is 68, the other is 90 - 68 or 22.
If I tell you one angle is x, the other must be 90 - x.