Sorry if this is not the right place for this sort of question, but I am at a lost. My niece has some summer homework, and neither of us have a clue how to solve this question. Its been too long since I was in school, and my math skills have plummeted since.

Could someone please show me how to solve this, with step by step, or even a link to a similar question that I could follow. I need to find the value of $s$.

$$\begin{align}
d &= s\cdot 3\\
c &= s\cdot 1.5\\
c &= 2\cdot d
\end{align}$$