Ok for the Rg it really long winded, are you familiar with binomial expansions (p+q) etc and ncr. You must form probability equations for r=1 (1 nitrogen 15) and r=2 (2N15) where n is the total glycine molecules. When the probability for r=2 is larger than for r=1, you know its more likely to have 2 N15 than 1N15. Thats all ill say though, kinda took ages trial and error etc.