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Answer: 2


A pyramid weighs $p$, a sphere weighs $s$ and a cube weighs $c$.

$2p + s = c$
scales$s + c = 3p$, but $c=2p+s$ so: $$\begin{align}s+(2p+s)&=3p\\
\Rightarrow\ 2s+2p&=3p\\
\Rightarrow\ 2s&=p\end{align}$$


This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.