Question

One road crew can pave a section of highway in 15 h. a second crew can do the same job in 10h. how long would it take to pave that same section of highway if both crews worked together?

Answer 1

For this case what you can do is an average to calculate the number of hours it will take them to work together:
 Let
 t: average time.
 hi: hours worked for each one.
 n: number of teams.
 The average will be given by:
 t = (h1 + h2) / (n)
 Substituting both values:
 t = ((15) + (10)) / (2) = 12.5
 answer
 it would take 12.5 h to pave that same section of highway if both crews worked together