A tin of oil was $\frac{5}{8}$ full. When 10 bottles of oil was taken out and 8 bottles of oil was poured into it, it was $\frac{3}{5}$ full. How many bottles of oil can the tin contain?
(a)20
(b)30
(c)40
(d)80
Answer
Answer (as printed): D
Explanation
Let $x$ be the number of bottles of oil that the tin can contain. Then, $\frac{5}{8} x-\frac{3}{5} x=10-8 \Leftrightarrow 25 x-24 x=2 \times 40 \Leftrightarrow x=80$.
Explanation as extracted from the printed page; notation may be imperfect.