Here is as far as I have gotten:

there are 3^10 = 59049 possible ways to fill out the exam answer sheet. I know that I need to add the probabilities of getting 6/10 correct + prob7/10 + prob8/10 + prob9/10 + prob10/10

i know that the probability of getting 10/10 by guessing is 1/59049, but I dont understand how to compute the probability of the others, can someone please point me in the right direction