rate and ratio?

　　　　　　　　　　　　　　　　　　　　　　　　
Question：
A box contains three kinds of coins: $5, $2 and $1
The ratio of the number of $5 coins, the number of $2 coins and the number of $1 coins is 4：5：3.
If the total value of the coins is$66, find the total number of $2 coins in the box.

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Solution：
（ The ratio of the number of $5 coins, the number of $2 coins and the number of $1 coins is 4：5：3 ）
Let 4k, 5k, 3k be the number of $5 coins, the number of $2 coins and the number of $1 coins respectively.

$5(4k) + $2(5k) + $1(3k) = $66
$33k = $66
k = 2

∴ The total number of $2 coins = 5(2) = 10

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