SAT Question e8f9e117: Linear Inequalities answer and explanation with Desmos solution

For the given circuit, the resistance $R$ is $500$ ohms, and the total potential difference $V$ generated by $n$ batteries is $6n$ volts. It's also given that the circuit is to have a current of no more than $0.25$ ampere, which can be expressed as $I\le 0.25$. Substituting $6n$ for $V$ and $500$ for $R$ in Ohm's law gives $I=\frac{6n}{500}$. Since $I\le 0.25$, it follows that $\frac{6n}{500}\le 0.25$. Multiplying both sides of this inequality by $500$ gives $6n\le 125$. Dividing both sides by $6$ gives $n\le 20.83$. Since $n$ must be a whole number (representing the number of batteries), the greatest number of six-volt batteries that can be used is $20$.