Let p be a probability function on set S where ... - JAMB Mathematics 1995 Question

Let p be a probability function on set S, where S = (a₁, a₂, a₃, a₄). Find P(a₁) if P(a₂) = (\frac{1}{3}), p(a₃) = (\frac{1}{6}) and p(a₄) = (\frac{1}{5})