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.)

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