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Combinatorics Difficulty 3.0 AMC 10/12 Find the answer United Kingdom

In a pirate's trunk there are 5 chests. In each chest there are 4 boxes and in each box there are 10 gold coins. The trunk, the chests and the boxes are all locked. Blind Pew unlocks 9 locks and takes all the coins in all the boxes he unlocks. What is the smallest number of gold coins he could take?

(A) $20$ (B) $30$ (C) $40$ (D) $50$ (E) $70$

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