a) R is the variable
b) a product, the variable is being multiplied
[tex]R=\frac{I}{PT}[/tex]Explanation
Step 1
[tex]I=\text{PRT}[/tex]the value for I depends on the value for R, it means, that I depends on the value for R
so, R is the variable
Step 2
when you have
[tex]\begin{gathered} I=\text{PRT} \\ is\text{ equal to} \\ I=P\cdot R\cdot T \end{gathered}[/tex]it is a multiplication between P, R and T,the variable is being multiplied
Step 2
solve for R
[tex]\begin{gathered} I=\text{PRT} \\ \text{you n}eed\text{ to do the inverse operation } \\ \text{Multiplication}\Rightarrow opposite\Rightarrow Division \end{gathered}[/tex]then, opposite operation is division, divide both sides by PT
[tex]\begin{gathered} \frac{I}{PT}=\frac{PRT}{PT} \\ \frac{I}{PT}=R \\ R=\frac{I}{PT} \end{gathered}[/tex]I hope this helps you