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.