\displaystyle P(A) = \sum_{i=0}^{50}\frac{50-i}{73}\frac{\binom{50}{i}\binom{150}{127-i}}{\binom{200}{127}} = \frac{\binom{200}{127}}{73}\sum_{i=0}^{50}(50-i)\binom{50}{i}\binom{150}{127-i} =