How many hours two persons take to finish one work ?

Question

In a factory, it takes a quality specialist 20 minutes to perform a damage-free checkup  of one machine. A temporary contractor takes 60 minutes to perform the same task.  How many hours does it take a quality specialist and a temporary contractor together to perform a damage free check of batch of 160 machines .?

Answer 1

For one machine:

t/20 + t/60 = 1

3t + t = 60

4t = 60

t = 15

 

Working together inspect one machine in 15 minutes.

 

How much time do they need for 160 machines?