What would have been the initial pressure reading of a sample of oxygen gas that started at 25.0 ml in volume and was compressed to 15.0 ml? the final pressure reading was 1.90 atm. (temperature was kept constant.)

The answer is 1.14 atm.

Use the Combined Gas Law on the reference tables to answer this question. You can ignore the temperature portion because it remains constant in this particular case.

25x = (15)(1.90)
25x = 28.5
x = 1.14 atm