Carrie can inspect a case of watches in two hours. jerry inspects a case in five hours. after inspecting a case for a half an hour, carrie takes a break. in the mean time, jerry takes over and completes the job, how long did this take him?

Write the hours as ratio

Carrie : Jerry
   2      :    5

Put this ratio in words, we say 'for every two hours Carrie takes to inspect a case, Jerry would need 5 hours'

Carrie has done 30 minutes on a case before taking a break. It means Carrie has 1.5 hours left on the case if she was to continue it. 

We need to work out the proportion of the time Jerry would take in comparison to 1.5 hours that Carrie would have taken.

First, we divide 2 by 1.5 ⇒ 

Then we divide 5 by 4/3 ⇒  hours

Jerry would have taken 3.75 hours to finish the rest of the case