Answer this question, and you could win a free Magoosh account. ðŸ™‚ We’ll have a new problem every month. You can see past challenge questions here.

Here’s what to do:

1. Submit an answer and explanation in the comments of this post by 2PM on Thursday, the 20th.

2. I will pick the comment with the best explanation, and this person will win a free account!

3. Comments will remain hidden until we release the answer and the winner, so don’t worry if your comment doesn’t show up immediately–we’ll still see it!

This post will be updated with the answer on Thursday, after the winner is announced. Good luck!

**UPDATE 3/20/14: Congrats to our winner, Marine! ðŸ™‚Â **

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Answer B.

If x represents the number of correct answers and y represents the number of incorrect answers and no answers, then the total number of points at the end of the test is 9x -7y. Since the final score is 0 then 9x = 7y. x and y represent answers so they are non negative integers. Therefore as 9 = 3 * 3 and 7 is prime, the lowest common multiple of 9 and 7 is their product: 9 * 7 = 63. In addition, we know that x + y < 30. Therefore, the only possible combination for (x,y) is (7,9). In this case, x + y = 16, which is lower than 30. The combination (14,18) would also be a solution for 9x = 7y but the sum is greater than 30. To conclude the number of questions on the test must be 16.

Hey Marine! You’re our winner because I thought your explanation was the most clear and concise. ðŸ™‚ I just emailed you with instructions to set up your free account!

Congratulations Marine. Good Explanation ðŸ™‚