Macro can finish the puzzle in 3 hours. We can say that her speed for working is 1/3 or 1/3 puzzle per hour.
Now if they work together, this turns into 1 puzzle per hour.
That must mean that Cliff would be doing the other 2/3 of the puzzle while Macro does his 1/3 in that hour. If Cliff has a speed of doing them 2/3 per hour. Then it would take 1 and a half hour to finish 1 whole puzzle.