Hello,

I feel like this should really be simple, but I'm still having trouble in finding the right equation for this word problem:

"John can finish a job in 9 hours. After working for 5 hours, he decided to take a rest. Randy helped out John and finished the job in 2 hours and 20 minutes. How long would it take for Randy to do the job alone?"

The answer to this is5 hours and 15 min. However, I don't quite understand how it got there.

For, the first few problems that I did with this type of word problem, I would either use,

this kind of formula:t/a + t/b=1,where t is the time spent working together, and a and b, their individual times.

or this formula:1/a+1/b= 1, where I take into account the rate of work of each individual.

Both types of formula has helped me get to the right answer, till I got to this problem.

What I did was to add the 5 hours from John and 2 hours and 20 minutes from Randy, in order to get the total time spent working, but I don't think that's right, since it's not yielding the right answer.

I hope someone can help, preferably in layman's terms. Thanks.