Jeff and kirk can build 75 feet retaining wall together in 10 hours. because jeff has more experience, he could build the wall by himself 1 hour quicker than kirk. how long would it take kirk to build the wall by himself.

Let J and K be the times of Jeff and Kirk respectively
To know the time it takes two people to perform a task together use the equation:

We also know from the problem that J=K-1, so

Now get common denominators


Take reciprocals




By the quadratic equation:
K=20.512
J=K-1=19.512
Putting these into the original formula for two people working together on a task

So it checks out.