application word problem

Question

Working together, Pat and Chris can reseal a driveway in 6 h. Working alone, Pat can reseal the driveway in 10 h. How long would it take Chris, working alone, to reseal the driveway?

Answer 1

1/6 = 1/10 +1/c

multiply by 30c

5c=3c +30

2c =30

c = 15 hours for chris working alone