The partial pressure of a gas, according to Dalton's law, is equal to the total pressure in the vessel times the fraction of the total pressure that gas makes up.
For this picture, it is helpful to imagine that each dot represents a mole of gas. We can count the moles to find out that we have 12 total moles of gas and 4 moles of orange gas.
We can assume the gases are behaving ideally, which means each mole of gas will contribute the same amount of pressure.
This means the fraction of pressure the orange gas makes up is:
Since the total pressure is 240 mmHg, the pressure of the orange gas is: