A Level maths help needed!!!

45 students
- 12 study biology
- 2 chemistry
- 3 physics
-3 biology and physics, 15 biology and chemistry
- 6 chemistry and physics
- 4 do all 3.

Caluclate that two students study one science,which is different to each other.

I am confused as on the mark scheme it saying multiple w/45 by y/44, but I don't understand why the denometer decreases

There are 45 students, right? The question asks you to calculate the *probability* that two students study one science which differ from each other.
The denominator decreases because at first, you're dividing by the total number of students available (45), hence the w/45. Once you find this student though, the total number of students decreases by 1, 45 - 1 = 44. So now, for your next value, you're dividing by the new total number of students available, i.e. 44.