A roofing crew can roof a house in 10 hours. If a second crew was added, the job would take 6 hours. How long would it take the second crew to roof the house alone?

Answer 1

Working together is like resistors working in parallel:

t1*t2 / (t1 + t2) = 6,
10t2 / (10 + t2) = 6,
Cross multiply:
10t2 = 60 + 6t2,
10t2 - 6t2 = 60,
4t2 = 60,
t2 = 60 / 4 = 15 h.