Answer:
We know that n = 50 and p =0.78.
We need to check the conditions in order to use the normal approximation.
Since both conditions are satisfied we can use the normal approximation and the distribution for the proportion is given by:

With the following parameters:


Step-by-step explanation:
Previous concepts
The binomial distribution is a "DISCRETE probability distribution that summarizes the probability that a value will take one of two independent values under a given set of parameters. The assumptions for the binomial distribution are that there is only one outcome for each trial, each trial has the same probability of success, and each trial is mutually exclusive, or independent of each other".
Solution to the problem
We know that n = 50 and p =0.78.
We need to check the conditions in order to use the normal approximation.
Since both conditions are satisfied we can use the normal approximation and the distribution for the proportion is given by:

With the following parameters:


630 red marbles
420 yellow marbles
210 purple marbles
because 1260 x 1/2 = 630, 1260 x 1/3 = 420, 1260 x 1/6 = 210
The probability of getting the correct order in the spelling bee is 1 chance in 362,880.
This is a permutation problem. Our goals is to find the total number of ways that the 9 students can be ordered.
For the first spot there are 9 choices, then 8 choices, then 7 choices....
We need to evaluate:
9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 = 362,880
Another way to write this is: 9!