Problem 5 · 2010 Math Kangaroo
Easy
Counting & Probability
careful-counting
Anna has connected every upper point to every lower point with straight lines. How many lines has she drawn?
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Answer: C — 30
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Hint 1 of 2
Count the dots on top and the dots on the bottom.
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Hint 2 of 2
Each top dot sends one line to every bottom dot, so think about repeated addition (or multiplying).
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Approach: multiply the two groups of dots
- There are 5 dots on top and 6 dots on the bottom.
- Each of the 5 top dots is joined to all 6 bottom dots, so we have 6 lines repeated 5 times.
- That is \(5 \times 6 = 30\) lines — the answer is C.
Mark:
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