a student takes a true false test that has 13 questions and guesses randomly at each answer. Let X be the number of questions answered correctly. Find P(fewer than 4)

Answer 1

cases:
one right
one right
two right
three right

find prob of each case and add them up

I will do one of them:
prob(exactly 3 of 13 right)
= C(13,3) (1/2)^3 (1/2)^10
= 286(1/2)^13
= 286/8192
= 143/4098