if sally can paint a house in4 hours and john can paint a house in 6 hours how l of them to paint the house together????
rate=1house/4 hours + 1house/6 hours
= 6/24 + 4/24=10/24 houses/hr
houses=rate*time
time=houses/rate = 1house/(10/24 house/hr)= 2.4 hrs.