Problem 3 · 2025 Math Kangaroo
Easy
Arithmetic & Operations
total-then-dividework-backward
A bookshelf with three rows has 17 books in the top row, 15 books in the middle row and 7 books in the bottom row. Monika would like to have the same number of books in each row, but she wants to rearrange as few books as possible. How many books does she have to move from the middle row to the bottom row?
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Answer: B — 2
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Hint 1 of 2
First find how many books each row should hold.
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Hint 2 of 2
Books should only be moved into rows that are short; figure out the bottom row’s shortfall.
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Approach: even out the rows with fewest moves
- Total books: 17 + 15 + 7 = 39, so each row should have 39 ÷ 3 = 13.
- The bottom row is short by 13 − 7 = 6 books; the top row has 4 spare and the middle has 2 spare.
- To move as few as possible, send the top’s 4 spare and the middle’s 2 spare straight to the bottom.
- So only 2 books go from the middle row to the bottom row.
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