But only then
Besides, funny thing is that anarchron answer does exactly what we have both suggested, just refraining from giving out the detailed solution.
Oh really? I'm sorry! I suppose the fact that we're both doing the same course study and method helps. I didn't quite understand what "Q", "c*" means.
And oh, I didn't even notice that Anachron's answer was wrong, Thanks for picking it up. I used 400g anyway ;-)