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Brainteaser: Fred’s weird teacher

By David, 18 September 2014 Brainteaser

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Fred’s teacher has a weird way of marking tests. He gives five marks for a right answer, but subtracts one for a wrong one.

Fred answered 20 questions on a test and got 70 marks. How many questions did Fred get right?

Scroll down or click here for the answer!

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Answer

There are lots of ways to do this question.

One important clue is that Fred’s mark was divisible by 5. The marks Fred got for correct answers must be divisible by 5, so the number of marks Fred lost for wrong answers must also be divisible by 5.

Then we can make a guess and check:
20 questions: 15 right + 5 wrong.
15 x 5 – 5 = 70.
So Fred got 15 questions right!

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