Please can you show me step by step what exactly it is you did? Because giving me nHe and so on isn't doing anything for me. :|
n
He and n
Kr ar just names of unknowns - as nj already wrote, they denote unknown number of moles of helium and unknown number of moles of krypton.
From PV=nRT we can calculate n, thus it is obvious what is sum of number of moles of helium and number of moles of krypon.
If you have n
He moles of helium its mass is 4*n
He - that's just molar mass times number of moles, nothing fancy.
Do the same for krypton, follow questions from my earlier post.