Find the volume then divide Over the rate of flow of water to get the time taken

Firstly, You have to find the total volume occupied in the tank.
To find the volume, Length x Breadth x height.

The question mentioned that the height of water is 30cm, length is 60cm and breadth is 40cm.
Hence the volume is 60 X 40 x 30 = 72 000 cm3

And then we have to keep in mind that there are 2 metal cubes inside too. They occupy the space too. Therefore, we have to find the volume of the 2 cubes.

A cube has 6 equal side. Hence is 1 side is 10cm, all sides are also 10cm. And again to find the volume, We multiply length x breadth x height. 10 x 10 x 10 = 1000cm3
There are 2 cubes, so we times 2, 1000 x 2 = 2000 cm3

Now we only want to find how much water is the tank. We will subtract the cube from the total.
72000cm3 -2000cm3 = 70 000cm3

So, the volume of water in the tank is 70 000 cm3.

If we have to drain 10L/min, how many minutes will i take to drain all the water?

1L is 1000cm3

Therefore, 70 000cm3 is 7L

70 Litres/10 = 7 min

OR

10L = 10 000 cm3
70 000cm3 divided by 10 000cm3 = 7 min.