Ah, ok, that makes more sense...
So...
Because 145mmHg and 355mmHg are pressures of the individual gases before they merged, these values are not partial pressures...And because the new volume would be enlarged, and therefore, with the same values of moles from the two gases, the partial pressures will be less... Right?
And thus, like AWK suggested...
P1V1 = P2V2 (1=He, 2=after)
P2 would be the partial pressure of He after the valve is open.
do the same for Ar, and add the partial pressure up to get the total pressure, which isn't 500mmHg.
Is that correct reasoning?