rate of pipe = 1/

*x*
rate of drain = -1/

*y*
A pipe can fill a tank in 4 hours if the drain is open

$\dfrac{1}{x} - \dfrac{1}{y} = \dfrac{1}{4}$ ← Eq. (1)

the pipe runs with the drain open for 1 hour

$\left( \dfrac{1}{x} - \dfrac{1}{y} \right)(1)$

If the pipe runs with the drain open for 1 hour and the pipe is then closed, the tank will be emptied in 40 minutes more

$\dfrac{1}{4}(1) - \dfrac{1}{y} \left( \dfrac{40}{60} \right) = 0$

$\dfrac{1}{y} = \dfrac{3}{8}$

From Eq. (1)

$\dfrac{1}{x} - \dfrac{3}{8} = \dfrac{1}{4}$

$\dfrac{1}{x} = \dfrac{5}{8}$

$x = 1.6 ~ \text{hrs}$ ← *answer*