Given information:
- The price elasticity of demand for good X is half the price elasticity of demand for good Y.
- A 25% rise in the price of good Y reduces its demand from 400 units to 300 units.

Calculating the price elasticity of demand for good Y:
Using the formula for price elasticity of demand:
Price elasticity of demand (PED) = (% change in quantity demanded) / (% change in price)

Given that the price of good Y increased by 25% and the demand decreased from 400 to 300 units, we can calculate the percentage change in quantity demanded:
% change in quantity demanded = ((300 - 400) / 400) * 100 = -20%

We know that the PED for good Y is twice that of good X. Let's assume the PED for good X is x.
So, the PED for good Y would be 2x.

Now, we can calculate the price elasticity of demand for good Y:
2x = (-20%) / 25% = -0.8
x = -0.4

Calculating the change in demand for good X:
Now that we know the price elasticity of demand for good X is -0.4, we can use the same formula to calculate the change in demand for good X when its price falls from 10 Rs to 8 Rs per unit.

% change in price = ((8 - 10) / 10) * 100 = -20%

Using the formula for price elasticity of demand:
% change in quantity demanded = (PED for good X) * (% change in price)
% change in quantity demanded = -0.4 * -20% = 8%

We know the initial demand for good X, but we don't know the initial price elasticity of demand. Therefore, we cannot determine the exact change in demand for good X. However, we can conclude that the change in demand for good X will be 8% given the price change.

Conclusion:
The change in demand for good X, when its price falls from 10 Rs to 8 Rs per unit, is approximately 8%.