Question

If Mr.Li can do 100 math problems an hour, and John can do 5 math problems an hour, what is the least amount of time it will take them to do 50 math problems together if each problem done by one of them has to be checked by the other (checking a problem takes as long as solving it in the first place)?

Answer 1

It takes John 12 min to do 1 problem (12= 60/5)
It takes Mr Li 0.6 min or 36 sec to check 1 problem (.6 = 60/100)

So it takes John 600 min to do 50 problems plus the 36 sec for Mr Li to check the last problem

total time = 10 hours 36 sec

Answer 2

Ah...I see....Thank you so much for your help!