Hours needed to paint garage

Question

Maria can paint a garage in 10 hours, while Sara needs 40 hours to paint the same type of garage. If Maria and Sara work together, how long would it take them to paint the garage? How fast would a third worker need to work if they wanted to get the entire job completed in 5 hours?

Answer 1

Think about how much gets done in one hour.

1/10 + 1/40 = 1/x

Once we find and apply the common denominator, 40x, we don't need it anymore.

4x + x = 40

5x = 40

x = 8 hours