Percentage over two baskets

Statement

I have two baskets of apples. In the first, $30\%$ of the apples are red; in the second, $15\%$ are red. The second basket contains four times as many apples as the first. What percentage of all my apples is red?

Solution

Let $n$ be the number of apples in the first basket; the second contains $4n$, for a total of $5n$.

Red apples. $0.30\,n + 0.15\cdot 4n = 0.30\,n + 0.60\,n = 0.90\,n.$

Percentage of the total. $\frac{0.90\,n}{5n} = 0.18 = 18\%.$ Check (with $n = 10$): first basket $10$ apples, $3$ red; second $40$ apples, $6$ red; $9$ red out of $50$ overall, i.e. $18\%$. Correct. $\boxed{18\%}$