How to calculate the mean molecular weight of the Sun I have a homework question in which I need to estimate a parameter known as $\beta_{P}$ and also the core temperature, both for the Sun.

However, prior to doing this I need to know the mean molecular weight of the Sun.

First question is, how would I go about doing this. 

Second question is, can I assume that it is about 70% hydrogen and 30% Helium?
For your second question, it depends upon whether you are talking about composition by number, or composition by mass (the numbers you give are roughly the composition by mass).  If you want to get really specific, you can add in the other elements too.

To answer the first, you add them up proportionally.