Question

Sally can paint a room in

8 hours while it takes Steve
9
hours to paint the same room. How long would it take them to paint the room if they worked​ together?

Answer 1

8.5 hours.  it's asking for an average

Answer 2

Sally's painting speed = 1 room / 8 hours

Steve's painting speed = 1 room / 9 hours

Combined painting speed = 1 room / 8 hours + 1 room / 9 hours

=> 1/9 + 1/8 = [8 + 9] / 72 = 17 / 72

So, they paint 17 / 72 parts of room per hour.

=> 1 room / x = 17 room /72 hours

=> x = 1 * 72 hours / 17 room = 4.235 hours.

You can verify how much each one has painted in 4.235hours

Sally: 1/8 * (72/17) = 9/17 parts of a room

Steve: 1/9 * (72/17) = 8/17 parts of a room

Sally + Steve = 9/17 + 8/17 = 17/17 = 1 room