$\begingroup$

It isn't all that hard mentally. As Ross Millikan says, $687=3\cdot 229$, so what you are looking for is the multiple of $229$ which is nearest to $33182\frac{2}{3}$.

The key to happiness is that $230$ is close to $229$, and it only has two digits.

Divide $3318.2$ by $23$, which gives you $143$. Multiply back by $230$, giving $32890$, and subtract $143$ to get $32747$, a multiple of $229$. Subtracting, you are $33182-32747=435$ short of $33182$, so add $229$ twice to get $33205$.

Before you feel too pleased with yourself, remember to multiply back by $3$, to get $99615$.