Approximating the amount of CO2 gas in a balloon from how it changes in water level when placed in water.

Question

Let's say I have a ballon with no CO2 in it and place it in a beaker of 10ml. It rises by 5 ml and the water level is 15ml. (this is the default measurement) Then I fill the balloon with x amount of CO2 gas tie it up and place it in the same beaker of 10ml. It rises by 15ml and the water measurement reads 25ml. The water level has changed by 10ml when compared with the default balloon measurement. How would I then approximate the amount of CO2 inside the balloon based on that change in water measurement level?