how is P(y is odd) calculated?

(B) Knowing that an odd number of tosses were necessary to observe a first result of six when tossing a fair die, find the probability that this first six occurred on the 3rd toss. Solution: Denoting by Y the number of tosses required to obtain a first result of 6, we have that Y ~ G(1/6). We use the previous result to find P(Y = 3) (1/6)(1 - 1/6) 275 P(Y = 3]Y is odd) = 0.2122 P(Y is odd) 1/2 - 1/6) 1296