text: # Math
  
  posted by on .
  
  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?
  
  • Math - ,
  
  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