Fuente TeX:

\mathbb{P}(N \geq 2) = \mathbb{P}(N=2; T=4) + \mathbb{P}(N=3; T=3)+ \mathbb{P}(N=4; T=2) \\ = \frac{C^4_2 \times C^5_4} {C^9_6} + \frac{C^4_3 \times C^5_3} {C^9_6} + \frac{C^4_4 \times C^5_2} {C^9_6} \\ = \frac{20}{21}