The question is based on Boyle's law, at a constant temperature , for a given mass of gas, Volume of a gas is inversely proportional to its pressure.
Mathematically it can be written as ;
1.02 atm x 225 mL = 2.99 atm x
Divide both the sides by 2.99
1.02 atm x 225 mL / 2.99 atm =
The volume of the gas decreases from 225 mL to 76.75 mL, on increasing the pressure from 1.02 atm to 2.99 atm.