Figure shows a potential divider circuit, which, by adjustment of the position of the contact X, can be used to provide a variable potential difference between the terminals P and Q.

What are the limits of this potential difference?

The lower limit is zero volt (0 V) when X is at the lower end of the 4kW resistor. The upper limit is the potential difference across the 4kW resistor when X is at the upper end of the 4kW resistor. That is $V=\left(\frac{25}{1K+4K}\right)4K=\left(\frac{25}{5}\right)4=20V$. Thus, the limits are 0 and 20 V.