Question

a tank can hold 40 litre of water 4/5 part is filled with water if 1/8 of water presnt in the tank is removed how much more water be added in order to fill the tank completly

Answer 1

filled with 4/5 part of water = 4/5 of 40 = 32 litres
1/8 part of water removed = 1/8 of 40  = 5 litres

Now water present = 27 litres

To fill completely 13 litres is required

y of 40 = 13

y = 13/40

So, 13/40 th part of water of 13 litres is required to fill the tank completely

Answer 2

Total capacity = 40 liters
4/5 is filled implies 
4/5x40 = 32 liters of water is currently present
out of 32 liters 1/8 is removed
32x1/8 = 4
water remaining in  the tank 32 - 4 = 28
water to be filled = total capacity - water currently present
ie 40-28 = 12
12 liters must be added to completely fill the tank