Another way of solving:

1. P(a) for one person = 1/4

2. P(a) for two persons P(A+B) = 1/4 + 1/4 - 1/4*1/4 = 8/16 - 1/16 = 7/16 < 0,5 ( A+B - AB, cause A or B, two independent nonexclusive)

3. P(a) for 3 persons = P(A+B+C) = 7/16 +1/4 - 7/16*1/4 = 44/64 - 7/64 = 37/64 > 0,5

Answer: 3 persons