Precal
posted by Lucy on .
What is the probability of getting an even number on a single roll of a number cube if you roll a 3 or greater.
My answer: is 1/3 because there are only two numbers that are even and greater than 3 on the number cube.
Teacher marked this as wrong. How did I mess this one up?

this sure looks like a "conditional" probability.
in general
P(A│B) = P(A AND B)/P(B)
read: P(A│B) as "the prob of A given B"
so let A be (even number)
let B be (greater or equal to 3)
so P(A and B) = 2/6 = 1/3
P(B) = 4/6 = 2/3
so P(A│B) = (1/3)/(2/3) = 1/2