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?

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