The probability of the k-th plane facing upwards is p (k) = c (50, k) (1/2) 50.

So the probability = ∑ c (50, k) (1/2) 50, where the sum of k is from 30 to 50.

= 1 14075475473 136/ 1 125899906842624=7 1297 172 1707 1/70368744 177664

=0. 10 13 1937553227032822 178443893790245056 15234375

This is calculated by computer.

Manual calculation, using approximate formula.

Average np=25 variance npq=25/2

It can approximate the normal distribution n (25,25/2)

p(N & gt； = 30)= F(x >； =√2)= 1-F(x & lt； √2)= 1-0.92 13=0.0787