To solve this we assume that the gas in the problem is an ideal gas. Then, we can use the ideal gas equation which is expressed as PV = nRT. At a constant temperature and number of moles of the gas the product of PV is equal to some constant. At another set of condition of temperature, the constant is still the same. Calculations are as follows:
P1V1 =P2V2
P2 = P1V1/V2
P2 = 10 psig x 30 ft^3 / 25 ft^3
P2 = 12 psig
Therefore, decreasing the volume from 30 ft^3 to 25 ft^3 at constant temperature would increase the pressure to 12 psig.
