Problem 4 · 2014 Math Kangaroo
Easy
Counting & Probability
careful-counting
A big square is made from 25 small squares put together. A few of the small squares have been lost. How many have been lost?
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Answer: D — 10
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Hint 1 of 3
A full big square is 5 rows of 5, which is 25 small squares.
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Hint 2 of 3
Don't try to count the holes — count the squares that are still there.
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Hint 3 of 3
Take the number that is still there away from 25 to find how many are missing.
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Approach: count the squares still present, then take that away from the full 25
- A whole big square is made of 5 × 5 = 25 small squares.
- Carefully count the small squares that are still in the picture: there are 15 of them.
- The lost ones are the ones missing from 25, so 25 − 15 = 10 squares were lost.
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