Solve this with out any rounding

Question

Working together, it takes two computers 4 minutes to send out a company email. If it takes the slower computer 20 minutes to do the job on its own, how long will it take the faster computer to do the job on its own

Answer 1

the combined workrate is 1/4

the slower work rate is 1/20

x + 1/20 = 1/4

20x + 1 = 5 <--- multiplies everything by 20

20x = 4

x = 4/20 = 1/5

The faster computer will take 5 minutes

Answer 2

f represents the time needed by the faster

4/20 + 4/f = 1

Can you solve for f and answer?