Okay, both RTR and GTC have 53 rares and 15 mythics.
Rares are on the sheet twice and mythics are on the sheet once.
So out of 121 cards on the sheet for a given set, 10 are shocklands, 2 of each. So the chance of getting a shockland is 10/121 = 1/12.1 and the chance of getting a specific...