Math problems

Helen can inspect a case of watches in 5 hours
i.e. In 1 hour, Helen can inspect 1/5 case.

Jim can inspect the same case of watches in 3 hours
i.e. In 1 hour, Jim can inspect 1/3 case.

Now Helen had worked for 1 hour, that means there are 4/5 case left.
So, time needed for Helen and Jim work together to complete the job
=(4/5) / (1/5 + 1/3)
= 3/2 hours.

Note: That 40-minute break is no use in calculation.

The working rate of Helen and Jim are 1/5 Box/hr and 1/3 box per hour.

After fisrt hour, 1/5 work done, 1-1/5 = 4/5 work left.

After lunch, the working rate = 1/5 + 1/3 = 8/15

Time need = (4/5) / (8/15)=4/5 x 15/8 = 3/2=1.5 hr