Dr. Pepper-
That’s a blast from the past!
My ninth-grade algebra teacher asked us that one many, many years ago (let’s just say sometime last century) and a certain wise-guy gave what turned out to be the correct answer, never suspecting he was actually correct.
Since:
a) I remember the original Q&A
b) I didn’t initially understand your question, so I googled it
…I can’t give the answer, but let me just drop a hint for others that not much math knowledge is needed, since this is a bit of a trick question.