Well, you're actually got more information than you need, as far as I can tell.

You are given that for a pure solution, you need 63.2mg; this means that you still need this amount of pure cysteine. However, now your weight is coming from a sample that is only 80% pure. This means that if you take 80% of the weight of the impure sample, you'll have to get 63.2mg. So, if 63.2mg if only 80% of the weight, how do you figure out what the full weight is? Just divide 63.2mg by 80% (0.

, and you'll have your total weight.