John can rinse them in 72 minutes by himself:

720/72 = 10 dishes per min the rate John

It will take his friend Bob 120 minutes to rinse these dishes:

720/120 = 6 dishes per min the rate of Bob

let 'x' represent the time John and Bob need to do 720 dishes together

10 + 6 = 720/x

by solving we find:

x = 45 min

click here to see the step by step solution of the equation:

John and Bob will need 45 minutes to wash the 720 dishes together.