Math Beyond Workbooks

Brain Teasers and Number Tricks

Answers



  1. Two. She first places three bars on each side of the scale. If they balance, she knows that the fake bar is one of the three not weighed. If they do not balance, she knows which group of three bars contains the fake. So for her second weighing, she takes the group of three that contains the fake and places two of these on the scale. If they balance, the fake is the bar not weighed. If they do not balance, the fake is the lighter bar.


  2. It can be done for $3 by cutting apart one of the groups of three links into three separate links, and using these to join the other four parts.


    3. fixed chain


  3. 25 square centimeters. There are several ways of arriving at this answer, which makes for a good discussion.


  4. 5050. This is quick and easy if you notice that 1 + 100 = 101, 2 + 99 = 101, 3 + 98 = 101, etc. Since there are 50 of these pairs that add up to 101, the problem becomes 101 x 50 = 5050.


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    Written by Ruth Estabrook
    restabrook@yahoo.com