Two computers take 30 seconds, so the time for both computers is 1/30 per second.
One computer takes 50 seconds so it is 1/50 per second.
2nd computer would be the speed of both computers minus the speed of the first computer: 1/30 - 1/50
Find common denominator: 30 *50 = 150
1/30 = 5/150
1/50 = 3/150
Subtract: 5/150 - 3/150 = 2/150 = 1/75
It would take the second computer 75 seconds
