Let's assume that the quantity of the original mixture is x litres. Then, the amount of alcohol in the original mixture is 2% of x, which is 0.02x litres.
When 10 litres of alcohol is added to the mixture, the total amount of alcohol becomes 0.02x + 10 litres, and the total volume of the mixture becomes x + 10 litres.
The new concentration of the mixture is 5%, which means that the amount of alcohol in the mixture is 5% of the total volume. Therefore, we can write:
0.02x + 10 = 0.05(x + 10)
Simplifying the equation, we get:
0.02x + 10 = 0.05x + 0.5
0.05x - 0.02x = 9.5
0.03x = 9.5
x = 9.5 / 0.03
x = 316.67
Therefore, the original quantity of the mixture was approximately 316.67 litres.