$A, S, M$ and $D$ are functions of $x$ and $y$, and they are defined as follows:
- $A(x, y) = x + y$
- $S(x, y) = x - y$
- $M(x, y) = xy$
- $D(x, y) = x/y$,
where $y \neq 0$.
A man travels three-fifths of distance $AB$ at a speed of $3a$, and the remaining at a speed of $2b$. If he goes from $B$ to $A$ and back at a speed of $5c$ in the same time, then:
- $1/a+1/b=1/c$
- $a+b=c$
- $1/a + 1/b=2/c$
- None of these