The total number of ways 2 students can be picked is 15 * 14 = 210 because you have 15 students to begin with and after you pick one, you're left with 14 more to choose from.
The total number of ways a senior and a junior can be picked is 8 * 7 = 56 because there are 8 seniors and 7 juniors.
The probability will then be 56 / 210 = 4 / 15. Hope this helps!
