Dalton's law states that the total pressure for a mixture of non-interacting gas is the sum of their individual partial pressures.
In this case, since the gases are helium and nitrogen, which are inert as well as non polar, we expect negligible interaction between the gases.
Thus, the total pressure is
[tex] 77mm Hg + 87 Torr \approx 77 Torr + 87 Torr = 164 Torr[/tex]
