=============================================
Athos, Porthos, Aramis and D'Artagnan are dividing n gold coins they received from the queen. Athos gets 25% of all the coins, then Porthos gets 25% of the remaining coins, then Aramis gets 25% of the remaining coins, then D'Artagnan gets 25% of the remaining coins. Still, there are some coins left, which they divide equally among four of them. If n is the least number of coins the queen could have given them, then how many coins did D'Artagnan get ?
A. 81
B. 189
C. 324
D. 405
E. 1024
==============================================I don't like names, so I'll set variables for the quantity of gold coins each of them gets
--> let a = count of coins initially apportioned to Athos
--> let b = count of coins initially apportioned to Porthos
--> let c = count of coins initially apportioned to Aramis
--> let d = count of coins initially apportioned to D'Artagnan
--> let k = count of coins left over after first allocation
We know the following:
--> a + b + c + d + k = n
--> a = .25n
--> b = .25(n - a) = .25(n - .25n) = .25(.75(n))
--> c = .25(n - b) = .25(.75(.75(n)))
--> d = .25(n - c) = .25(.75(.75(.75(n))))
--> k = n - d -c - b - a = (.75)(.75)(.75)(.75)n
We also know:
--> d is an integer
--> k is an integer
--> It is possible to divide k equally among the 4
Let's start with d:
\(d = .25(.75(.75(.75(n))))\)
\(d = \frac{1 * 3 * 3 * 3}{4 * 4 * 4 * 4}n\)
\(d = \frac{27}{256}n\)
Note that 27 and 256 are coprime. So n has to be some multiple of 256. So there exists some integer q such that n = 256q
Then let's think about k.
\(k = .75(.75(.75(.75(n))))\)
\(k = \frac{81}{256}n\)
Again, 81 and 256 are coprime, but we already knew n was a multiple of 256.
However, we know that 4 divides k. This means that q itself must be a multiple of 4; there exists some integer x such that 4x = q
\(k = \frac{81}{256}n\)
\(k = \frac{81}{256}(256q)\)
\(k = \frac{81}{256}(256(4x))\)
\(k = 81(4x) = 324x\)
Now let's revisit what this 4x = q means for d.
\(d = \frac{27}{256}n\)
\(d = \frac{27}{256}(256(4x))\)
\(d = 27(4x) = 108x\)
Now D'Artagnan's final allotment is \(d + \frac{k}{4}\).
We can rewrite this as \(108x + \frac{324x}{4} = 108x + 81x = 189x\)
This is minimized by setting x = 1.
In which case D'Artagnan gets 189 coins.
----------------------------------------------