You know that you can substitute numbers here, however you have to be careful not to be misled because the answer that they are looking for is easier to derive in this one particular case using variables.  Let’s say we start with x marbles.  m marbles are added yielding t times the original number..x. Thus we have x + m = tx. We can say m = tx – x.  If we factor x out of the expression on the right, we have m = x(t – 1).  Dividing both sides by t – 1 yields m/t – 1 = x.  Therefore E is the correct answer.
 

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