How to calculate the concentration of the diluted solution given a source concentration in ppm?
Let us assume for a moment, that the parts-per-notation is actually unambiguous. We then know that we can convert the concentration given in ppm $c^\dagger$ into the concentration $c$ in SI units $\mathrm{mol\,L^{-1}}$, or $c^\dagger\propto c$. Then we know that the concentration is defined as the amount of substance per volume of solution.
$$c = \frac n V$$

When diluting a solution the amount of substance stays the same, hence we have two equations, that we can combine.
$$\begin{align}
  c_0 &amp;= \frac{n}{V_0}  &amp;&amp;\text{source solution}\tag1\\
  c_\mathrm{dil} &amp;=\frac{n}{V_\mathrm{dil}}  &amp;&amp;\text{diluted solution}\tag2\\
  c_\mathrm{dil} &amp;=\frac{c_0V_0}{V_\mathrm{dil}}  &amp;&amp;\therefore\tag{1=2}
\end{align}$$

Which is exactly the formula you used (when I assume that $M$ stands for molarity).

Ergo your calculation result, as orthocresol already pointed out, of $c^\dagger_\mathrm{dil} = 18.7~\mathrm{ppm}$ is correct.

The error therefore has to be in the question, or in copying the same.