Question

Sally can paint a room in 9 hours while it takes Steve 3 hours to paint the same room. How long would it take them to paint the room if they worked together?

Answer 1

2 1/4 hours

Explanation:

Okay, so- The first thing I did was see how much they got done per hour. Like this:

Sally ⇒

3 hours = 1 room

1 hour = 1/3 of the room

Steve ⇒

9 hours = 1 room

1 hour = 1/9 of the room

Sally + Steve ⇒

1 hour = 1/3 + 1/9 = 3/9 + 1/9 = 4/9

So, they get 4/9 of a room done in 1 hour.

Number of hours needed ⇒

1 ÷ 4/9 = 2 1/4

Answer: They need 2 1/4 hours to paint the room.

Answer 2

You can solve this one by adding their rates, but you have to invert the numbers they give you to find the rate. Rate is measured in something per unit of time, and "per" indicates division. So the rates are: 1/3 of an hour (Sally) and 1/6 of an hour (Steve). So add the rates: 1/3 + 1/6 = 1/2. (1/3 is 2/6, and 2/6 + 1/6 = 3/6). Since their combined rate is 1/2 of a room in one hour, it takes them two hours to paint the room.

This answer makes sense: It takes Sally three hours by herself; with Steve's help, she's faster, but not THAT much faster, because he's pretty slow. You can often spot-check word problem answers like this by giving it a common sense once-over. If we came up with four hours, that would clearly be wrong: it should take her LONGER if she's getting help.