Hmm, I think you would take 40 and multiply it by 2 because her sister used "twice as many". Then I'm pretty sure you would take your answer from that and divide it by 2 because she used that amount of paper in "half the number of days"?
40 x 2 = 80 all together used
80 / 2 = 40 how many used each day
Is that what you were asking? If I'm wrong let me know please
