A sealed vessel containg helium, chlorine and fluorine has a total pressure of 205 kpa and partial pressures of 15 kpa for chlorine and 89 kpa for fluorine. what is the pressure for helium in kpa

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