Mozilla_test.sagews

p_null = (74*0.55 + 48*0.64) / (74+48)
show(p_null)

$\displaystyle 0.585409836065574$

comb(n, k) = factorial(n) / (factorial(n-k)*factorial(k))
print p_null^31

#print comb(31,48) * p_null

6.18387170244974e-8