Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Guesstimate Time

As we go about our daily business, we are constantly making decisions based on guesses and estimates. This exercise will help improve your ability to make educated guesses about time.

When you are estimating each answer, try to pay attention to how you are solving the problem. Are you guessing the whole number or are you trying to calculate it based on smaller guesses? Avoid using a calculator or pen and paper until after you have made your guestimate.

1. How much time per year do you spend on the Internet?
2. How much time per month do you spend in a car?
3. How long would it take you to count to a million?
4. If you were a bird, how long would it take you to fly to the North Pole?
5. How long does it take to fill up your bathtub?

With practice, you will get better and making estimates by making quick approximations and simplification.
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For more mind exercises: http://www.braingle.com/mind/