Think of a number. Multiply it by 2 and add 4. Multiply the result by 0.5 and add 3, and then subtract the original number. I know what number you have left. What must it be?

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Think of a positive integer. Multiply it by 5 and add 2. Multiply the result by 2 and subtract 7. I know the last digit of the number you have now. What is it?

Excel in math and science

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It's hard to learn from lectures and videos

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Think of any integer between 1 through 9 inclusive. Multiply it by 2 and add 1. Then multiply the result by 5 and add 3. I know the last digit of the number you ended up with. What is it?

Excel in math and science

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It's hard to learn from lectures and videos

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Think of a number. Multiply it by 3 and subtract 1. Divide that by 0.5 and add 7. Subtract 6 times the original number. I know what number you have left. What must it be?

Excel in math and science

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It's hard to learn from lectures and videos

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I am thinking of a whole number. I multiply it by 4 and add 6. Then I multiply the result by 0.5 and add 2, to end up with 9. Can you guess the number I first thought of?

Excel in math and science

Master concepts by solving fun, challenging problems.

It's hard to learn from lectures and videos

Learn more effectively through short, conceptual quizzes.

Our wiki is made for math and science

Master advanced concepts through explanations,
examples, and problems from the community.

Used and loved by 4 million people

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including olympiad champions, researchers, and professionals.

Your answer seems reasonable.
Find out if you're right!