According to Dalton's law of partial pressure
The total pressure of a mixture of ideal gases is sum of the partial pressures of each gas present in the mixture
So if we know the partial pressure of all the gases we can determine the total pressure of the mxiture by adding all of them.
Thus
Ptotal = PCl2 + PF2 + PHe
Thus
Partial pressure of helium = 205 - ( 89 + 15) = 101 kPa
