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With this problem, you started on the right track. Make sure the binomial formula includes p^x and q^(n-x).

p = probability of success

q = 1-p

You need to think how "success" is defined in this problem - it's the probability of getting 1 question correct by guessing the correct answer out of 5 choices.

Then you need to find the probability of getting 0 correct, 1 correct, 2 correct, and 3 correct. Add these up, then subtract this sum from 1.

In a binomial probability distribution (any distribution, for that matter), the sum of all the probabilities equal 1. So, the probability of getting 4 or more correct is equal to 1 minus the probability of getting 3 or less.