Question 280103
We have to assume there are two places with easy access to the same dishes and the two workers don't get in each other's way.
Even 80 minutes seem to be excessively long to rinse dishes. We are not talking about washing them but just rinsing them.

x/80+x/120=1
3x+2x=240
5x=240
x=240/5
x=48 minutes together

another interesting way to calculate using the 480 dishes
in 1 minute john can do 480/80=6 dishes
in 1 minute bob can do 480/120=4 dishes
4x+6x=480
10x=480
x=48 minutes