Find the valency of the element named in each case.

Step 1. (a) Four hydrogens: $4 \times 1 = 4$, so C is $4$.

Step 2. (b) Three chlorines: $3 \times 1 = 3$, so Al is $3$.

Step 3. (c) and (d) One oxygen ($2$) for one copper: $2$. One oxygen for two coppers: $\dfrac{2}{2} = 1$. Copper has two valencies.

Step 4. (e) $\dfrac{3 \times 2}{2} = 3$.