one solution is 4 total students with 3 having blue eyes but there is more than one answer.

For the general solution, let x be the total number of students and let b be the number of students with blue eyes.

P(2b) = .5

P(2b) = b/x * (b-1)/(x-1) with b ≤ x and b,x ∈ Z⁺

This general solution can be solved numerically, giving 4 answers is you restrict x to be less than 100000.