[kemma]

How can you calculate number-average moleculear weight, if you only know that Polypropylene sample is in 4 fraction and the masses and molar masses of those fractions are
mass/g          molar mass/g/mol
50                   75 000
100                100 000
100                200 000
30                 400 000

[MOTOBALL]

I think that your first step should be to look up the definition of number-average mol. wt.

Second step follows from first.