Think about it this way:
By saying 3/4 x 4/5 you are saying 3/4 OF 4/5
So you can make a problem where 4/5 of something is leftover (perhaps in food? Or a job?) and say a person finished 3/4 of that leftover whatever. The question would be: how much of the WHOLE thing did the person finish?
Ex/
Bob and Jon are arranging books from a box. Jon arranges 1/5 of the books, leaving the rest to Bob. Bob arranges 3/4 of the rest. How much of the whole box of books did Bob arrange?
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So in this problem, Jon arranged 1/5 of the books. Which means, the “rest” is 4/5 of the whole box left for Bob.
Bob decides to only 3/4 OF that 4/5 that is left for him.
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The question asks: How much OF the whole box of books did Bob arrange?
To find that, you would do 3/4 x 4/5.
3/4x4/5 = 12/20 and when simplified, (divide the numerator and denominator by 4)
=3/5
That will be the answer. Bob arranges 3/5 of all the books.
Hope this helps!!