If a pencil factory makes 50 cases of pencils in 5 hours. Let's first find out how many cases are made per hour.
50 ÷ 5 = 10 cases per hour
Now you need to know how long it will take to make 55 cases. Since we already know that it takes 5 hours to make 50 cases. So you need to know how many cases for the extra 5 cases. If it takes 1 hour to make 10 cases, it would only take 1/2 hour to make 5 cases.
So it would take 5 1/2 hours to make 55 cases.
Hope this helps. :)