# 2020 CIME II Problems/Problem 8

## Problem 8

A committee has an oligarchy, consisting of $A\%$ of the members of the committee. Suppose that $B\%$ of the work is done by the oligarchy. If the average amount of work done by a member of the oligarchy is $16$ times the amount of work done by a nonmember of the oligarchy, find the maximum possible value of $B-A$.

## Solution

Average work done sets up an equation: $$\frac{B}{A} = 16\frac{100-B}{100-A}$$ $$(100-A)B = 16(100-B)A$$ $$100B - AB = 1600 A - 16AB$$ Let $B-A = C$ and $A+B = D$: $$50C + 50D - \frac{D^2-C^2}{4} = 800D - 800C - 4(D^2-C^2)$$ $$15D^2 -3000D = 15C^2-3400C$$ Complete the squares: $$15(D-100)^2 - 1500^2 = 15(C-1700/15)^2 - 1700^2/15$$ $$(D-100)^2 + \frac{(17^2-15^2) \times 100^2}{15^2} = (C-1700/15)^2$$ $$C = \frac{1700}{15} \pm \sqrt{(D-100)^2 + \frac{8^2 \times 100^2}{15^2}}$$

Note that $\frac{1700+800}{15}>100$ so must use minus. This means that C is maximized if $D=100$ $$C = \frac{1700}{15} - \frac{8 \times 100}{15} = 900/15=60$$ $B-A$ is at a maximum $60$

## Solution 2

As in the first solution, we get $100B - AB = 1600A - 16AB$. We rearrange and obtain $15AB + 100B - 1600A = 0$. We divide by $15$ to obtain $AB + \frac{100}{15}B - \frac{1600}{15}A = 0$. We then subtract $\frac{160000}{225}$ from both sides, and factor to obtain $(A + \frac{100}{15})(B - \frac{1600}{15}) = -\frac{1600000}{225} = -\left(\frac{400}{15}\right)^2$. If we graph this with $A$ being on the $x$-axis and $B$ being on the $y$-axis, this equation is the hyperbola $xy = 1$, except scaled up by $\frac{400}{15}$ and translated $\frac{100}{15}$ to the left and $\frac{1600}{15}$ up. This graph intersects $(0, 0)$ and $(100, 100)$, and the maximum difference clearly occurs at the point when the slope of the function is $1$. This is at $(-\frac{100}{15} + \frac{400}{15}, \frac{1600}{15} - \frac{40}{15}) = (20, 80)$. Our answer is $\boxed{60}$.

~mathboy100

## Solution 3

Assume WLOG, there are $100$ members and that there are $100$ work that is done. So, there are $A$ members of the oligarchy and they've done $B$ work in total. Thus, on average, each member of the oligarchy does $\dfrac{B}{A}$ work. Then, the average work a nonmember of the oligarchy will be $\dfrac{B}{16A}$. Thus, $$B+(100-A)\cdot \dfrac{B}{16A} = 100$$ $$\implies 16AB+(100-A)B = 1600A$$ $$\implies 3AB+20B-320A = 0$$ $$\implies (3B-20)(3A+20) = -6400$$ When $3B-20 = -80$ and $3A+20 = 80$, we have $A=20, B=80$, giving $B-A = 80-20 = \boxed{60}$

~sml1809