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Average speed and percentage

By Murray Bourne, 07 May 2009

This kind of puzzle always catch out students.

I travel to work at 30 km/h, but on the way home there is less traffic so I can travel at 60 km/h. What is my average speed?

And the second one is also an old favourite.

I have a jar of water and I remove 10% of it. Then my friend comes along and increases the amount of water by 10%. Is there more, the same or less water in the jar than when we started?

I'll offer a prize to the first correct answers with good reasoning. What prize?

You will get a warm inner glow for having achieved something. 🙂

See the 10 Comments below.

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To enter math, you can can either:

  1. Use simple calculator-like input in the following format (surround your math in backticks, or qq on tablet or phone):
    `a^2 = sqrt(b^2 + c^2)`
    (See more on ASCIIMath syntax); or
  2. Use simple LaTeX in the following format. Surround your math with \( and \).
    \( \int g dx = \sqrt{\frac{a}{b}} \)
    (This is standard simple LaTeX.)

NOTE: You can mix both types of math entry in your comment.


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