time to mow lawn

Question

Jim takes 30 minutes to cut the lawn. When Bob helps him it takes 20 Minutes.  How long would it take for Bob to mow the lawn?

Answer 1

Let's imagine the lawn is broken up into 30 pieces.

Jim completes 30 pieces in 30 minutes, or 1 piece per minute. (30 / 30 = 1)

J + B complete 30 pieces in 20 minutes, or 1.5 pieces per minute (30/20 = 1.5)

1.5 - 1 = .5

 

So Bob completes half a piece a minute, or 1 piece every 2 minutes

2 ^min/_piece * 30 pieces = 60 min

Bob will take 60 minutes to mow the lawn on his own.