The statement

tells us how to set up the differential equation.the rate of change of the height of the free surface level is directly proportional to the square root of the height

It tells us that the rate of change of the height is directly proportional to the square root of the height

Thus, our DE is , where is some constant of proportionality.

To solve, apply the separation of variables technique:

Integrating both sides, we see that

Use the first condition to find C.

Then use the second condition to find k.

Finally, find the time it takes to reach a height of 1.25.

Can you take it from here?

--Chris