A 20.0-liter flask contains a mixture of argon at 0.72 atmosphere and oxygen at 1.65 atmospheres. What is the total pressure in the flask?

The total pressure of the flask is sum of partial pressures of the gases present in a mixture of gas

This is Dalton's law of partial pressures.'

say we have a mixture of "n" gases with respective pressure : P1, P2 ... Pn

total pressure of the gases = P1 + P2 + P3 + ... Pn = Ptotal

So in the given problem the total pressure of flask will be = 0.72+1.65 = 2.37 atm