Machine A was able to finish copying a set of document in 36 minutes. Machine B was brought in to work together with machine A to finish copying the document in 20 minutes. How long would it take machine B to finish copying the document by itself

Answer 1

A's rate ---- 1/36
B's rate ----- 1/x
combined rate = 1/36 + 1/x
= (x+36)/(36x)

1/( (x+36)/(36x) ) = 20
36x/(x+36) = 20
36x = 20x + 720
16x = 720
x = 45

it would take 45 min for B alone

Answer 2

just add up the amount of work done by A and B in a minute. Together, they do 1/20 of the job:

1/36 + 1/x = 1/20