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Summer Clouds moderators

24/06/2017 at 10:16
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I have exactly ten coins whose total value is $1. If three of the coins are quarters, what are the remaining coins?




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    Phan Thanh Tinh Coordinator 24/06/2017 at 13:18

    1 dollar,1 penny,1 nickel,1 dime,1 quarter worth 100 ; 1 ; 5 ; 10 ; 25 in cents respectively.

    The remaining coins have : 10 - 3 = 7 (coins)

    Their total value is : 100 - 25 x 3 = 25 (cents)

    Hence,each remaining coin must worth less than 25 cents.So,there can be 3 kinds of coins : pennies,nickels,dimes.

    Let p,n,d be the number of coins of each kind\(\left(p,n,d\in N\right)\)

    We have :\(\left\{{}\begin{matrix}p+n+d=7\\p+5n+10d=25\end{matrix}\right.\)\(\Rightarrow4n+9d=18\)

    \(\Rightarrow n=\dfrac{18-9d}{4}=9\times\dfrac{2-d}{4}\) but \(n\in N\)

    \(\Rightarrow2-d⋮4\Rightarrow d=2\)\(\Rightarrow\left\{{}\begin{matrix}p+n+2=7\\p+5n+20=25\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}p+n=5\\p+5n=5\end{matrix}\right.\)

    \(\Rightarrow4n=0\Rightarrow n=0\Rightarrow p=5\)

    Hence,the remaining coins include 5 pennies and 2 dimes


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