Could you explain how you did those? I tried doing it with the formula that my teacher gave me (time (rate 1 + rate 2) = job still to do) but I was getting all sorts of weird answers, not simple ones like you ended up with.

Edit: I checked the back of my textbook, and it said the answers were Maria= 34.1 minutes and Johannes= 30.1 minutes. But I have no idea how it got those answers.