Official Solution:
On a market, two merchants sold total of 100 pineapples and earned the same amount of money. If the first merchant had sold the second merchants pineapples at first merchant's price per pineapple, he would earned \($15\) and if the second merchant had sold the first merchants pineapples at second merchant's price per pineapple, he would earned \($6\frac{2}{3}\). How many pineapples did the second merchant sell ?
A. \(20\)
B. \(30\)
C. \(40\)
D. \(60\)
E. \(80\)
Say the first merchant sold \(x\) pineapples at \($a\) per pineapple and the second merchant sold \(y\) pineapples at \($b\) per pineapple.
They earned the same amount, so: \(xa = yb\). This gives: \(\frac{a}{b} = \frac{y}{x}\).
If the first merchant had sold the second merchants pineapples at first merchant's price per pineapple, he would earned \($15\): \(ya=15\);
If the second merchant had sold the first merchants pineapples at second merchant's price per pineapple, he would earned \($6\frac{2}{3}\): \(xb=\frac{20}{3}\)
Divide: \(ya=15\) by \(xb=\frac{20}{3}\) to get: \(\frac{y}{x}*\frac{a}{b}=\frac{9}{4}\)
Substitute \(\frac{a}{b} = \frac{y}{x}\) into \(\frac{y}{x}*\frac{a}{b}=\frac{9}{4}\) to get \(\frac{y}{x}*\frac{y}{x}=\frac{9}{4}\);
\((\frac{y}{x})^2=\frac{9}{4}\);
\(\frac{y}{x}=\frac{3}{2}\);
Since \(x+y=100\), then \(x=40\) and \(y=60\).
Answer: D