Answer:
By the time that Molly does 40 push-ups, Liza would have done 60.
This is solved simply by multiplying 15 by 4.
How did we get "15 x 4?"
Well, to get Molly's number of push-ups, we would have to see how many times Molly's original number, 10, could go into the end result, 40.
To get that, we would have to divide 40 into 10; which is 4.
So, we would multiply Liza's original number, 15, by 4.
Which would equal 60.
Hope this helps! <3