Well, let's work backwards starting with our 15. What did we do to the previous number to get 15? We subtracted 5. So, we'd then perform the opposite operation, in this case it would be addition rather than subtraction. We'd then end up with 20. Now we have one more step. What did we do to the previous number to get twenty. As the question states, we mulitplied by four. Hence, we'd divide by four. The answer to your question is 5. Let's check.
$${\mathtt{5}}{\mathtt{\,\times\,}}{\mathtt{4}} = {\mathtt{20}}$$
$${\mathtt{20}}{\mathtt{\,-\,}}{\mathtt{5}} = {\mathtt{15}}$$ Voila!
Well, let's work backwards starting with our 15. What did we do to the previous number to get 15? We subtracted 5. So, we'd then perform the opposite operation, in this case it would be addition rather than subtraction. We'd then end up with 20. Now we have one more step. What did we do to the previous number to get twenty. As the question states, we mulitplied by four. Hence, we'd divide by four. The answer to your question is 5. Let's check.
$${\mathtt{5}}{\mathtt{\,\times\,}}{\mathtt{4}} = {\mathtt{20}}$$
$${\mathtt{20}}{\mathtt{\,-\,}}{\mathtt{5}} = {\mathtt{15}}$$ Voila!
