>>23601521They are using the probability mass function for the binomial distribution, which is totally unnecessary if you're using the complement rule.
So using the complement rule, the direct solution is a simple 1-0.7^3.
If you want a more direct solution, it would be
sum_{i=1}^3 3Ci * 0.3^i *(1-0.3)^(3-i)
Gary's solution was neither direct nor computationally simple. But then again, not wrong.
