probibility of at least 2 girls in 7 births
= 1 - p of less than 2 girls in 7 births
= 1 - of 0 girls - p of 1 girls
=1 - [7 C 0](0.5)^0(0.5)^7 - [7 C 1](0.5)^1(0.5)^6
=1 - (0.5)^7{[7 C 0] + [7 C 1]}
=1 - (0.5)^7(1 + 7)
= 1 - 0.0078125 * 8
= 1 - 0.0625
= 0.9375
hope this helps
