According to Dalton's Law of Partial Pressure, the total pressure of a mixture of gasses is the sum of the partial pressure provided by each. To determine the partial pressure, you need to know the total number of moles of gas pressent. According to the ideal gas law, the total number of moles of gas is what creates the pressure. Once you know the total number of moles, you can determine what percent each gas contributes by taking ratio of that gas's moles to the total. The total pressure multiplied by the proportion from each gas will provide the partial pressure.
Moles of H2 = 40g(1mole/2.00g)= 20 moles
Moles of O2 = 50g(1mole/32g) = 1.6 moles
Moles of Ar = 25g(1mole/39.95g) = 0.63 moles
Total moles of gas = 22.23 moles
%H2 = 20/22.23 = 90% Partial Pressure of H2 = 90% * 875mmHg = 787 mmHg
%O2 = 1.6/22.23 = 7.2% Partial Pressure of O2 = 7.2%*875mmHg = 63.0 mmHg
%Ar = 0.63/22.23 = 2.8% Partial Pressure of Ar = 2.8%*875mmHg = 24.8 mmHg