Question 65129
First, find the rate (job per minute) of each computer.

If one computer can solve the problem in 75 minutes, then it can solve 1/75 of the problem in 1 minute.
If the second computer can solve the problem in 50 minutes, then it can solve 1/50 of the problem in 1 minute.

Together, the two computers can solve {{{(1/75 + 1/50)}}} of the problem in 1 minute.
{{{1/75 + 1/50 = 2/150 + 3/150}}} = {{{5/150 = 1/30}}}

If the two computers can solve {{{1/30}}} of the problem in 1 minute, then it will take them 30 minutes to solve the whole problem working together.