Question

Jenny has just started working on a 250 piece jigsaw puzzle. She usually takes 3 hours to work a puzzle of that size. Her little brother wants to help. He could put the puzzle together in 6 hours. How much time would the two need to work the puzzle together? (Round your answer to the nearest tenth.)

Answer 1

You need to figure out the rates at which they both work. 

For Jenny, its 250 pieces / 3 hours = 83.3 pieces / hour 
For her brother, its 250 pieces / 6 hours = 41.7 pieces / hour 

so together, that adds up to 125 pieces / hour. 

(125 pieces / hour) = (1/125) hours / piece 
(1/125) hours / piece * 250 pieces = 2 hours because the pieces on the top and bottom cancel 

so the answer is 2 hours

Answer 2

Jenny: 250 pieces / 3 hours = 83.3 pieces per hour

Brother: 250 pieces / 6 hours = 41.7 pieces per hour

83.3 + 41.7 = 125 pieces per hour total

250 pieces / 125 = 2 hours total