Working together jill and bill can tar a roof in 6.67 hours. had he done it alone it would have taken bill 15 hours how long would it take hill to do alone?

Found a simple equation.
h = (a * b) / (a + b)

h = total hrs with both.
a = total hrs Bill.
b = total hrs Jill.

6.67 = (15 * b) / (15 + b)
6.67(15 + b) = 15b
100.05 + 6.67b = 15b
100.05 = 8.33b

b = 10005 / 833 or 12.0108 hrs