A boy can rake a yard in 2.5 hours, his sister can rake it in 4 hours. How long will it take both of them to rake it?
boy's rate = 1/2.5 or 2/5
sister's rate = 1/4
combined rate = 2/5 + 1/4 = 13/20
time at combined rate = 1/(13/20)
= 20/13 hrs or appr 1.54 hrs or 1 hour and 32 minutes