Two pipes $\text{A}$ and $\text{B}$ are attached to an empty water tank. Pipe $\text{A}$ fills the tank while pipe $\text{B}$ drains it. If pipe $\text{A}$ is opened at $2 \; \text{pm}$ and pipe $\text{B}$ is opened at $3 \; \text{pm},$ then the tank becomes full at $10 \; \text{pm}.$ Instead, if pipe $\text{A}$ is opened at $2 \; \text{pm}$ and pipe $\text{B}$ is opened at $4 \; \text{pm},$ then the tank becomes full at $6 \; \text{pm}.$ If pipe $\text{B}$ is not opened at all, then the time, in minutes, taken to fill the tank is

$\therefore$ The time taken by $\text{A}$ to fill the tank if $\text{B}$ is not opened at all $ = \dfrac{ \frac{12x}{5} \; \text{litres}}{x \; \text{litres/hour}}$