Problem 4 · 2018 Math Kangaroo
Easy
Counting & Probability
careful-counting
Leonie has one stamp for each of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. She uses them to stamp the date of the kangaroo competition (see picture). How many of the stamps does Leonie use?
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Answer: B — 6
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Hint 1 of 2
Write out the whole date as it is stamped: 15 03 2018.
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Hint 2 of 2
She has only one stamp of each digit, so a digit that shows up twice still uses just one stamp — count the different digits.
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Approach: write the date and circle the different digits, counting each kind once
- The date is stamped as 1 5 0 3 2 0 1 8.
- Cross out repeats: the 0 and the 1 each appear twice, but she only owns one stamp of each.
- The different digits are 0, 1, 2, 3, 5, 8 — that is 6 stamps, answer B.
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