If the manometer reading is 305 mm and the atmospheric pressure is 1.03 atm, what is the pressure of the gas in the flask?

We need to make sure that the pressure is in the same units.

let's convert the atm to mmHg. remember that 1 atm = 760 mmHg

1.03 atm (760 mmHg / 1 atm) = 783 mmHg

to find the pressure of the gas, you just need to subtract the values

pressure of gas = 783 - 305 = 478 mmHg