math
posted by frank on .
If Sally can paint a house in 4 hours, and John can paint the same house in 6 hour, how long will it take for both of them to paint the house together?

work=rate*time
worktotal=sallyrate*time + johnrate*time
time= worktotal/(sallyrate +johnrate)
= 1house/(house/4 + house/6)
= 24/24 * 1/(1/4 + 1/6)
= 24 * 1/(6 + 4)= 24/10 hrs
check my thinking.