Bunuel wrote:
Tough and Tricky questions: Probability.
Kate and Danny each have $10. Together, they flip a fair coin 5 times. Every time the coin lands on heads, Kate gives Danny $1. Every time the coin lands on tails, Danny gives Kate $1. After the five coin flips, what is the probability that Kate has more than $10 but less than $15?
(A) 5/16
(B) 1/2
(C) 12/30
(D) 15/32
(E) 3/8
total number of possible outcomes when coin will be flipped 5 times= 2^5=32
now, for kate to have more than $10, she must have min. 3 favorable and max. 4 favorable draws in the 5 throws. (because if she wins in all the 5 draws, then she will have $15)
kate will win, every time tails (T) appears on the coin.
case 1) kate wins in 3 draws and losses in two. so we have 3 T's and 2H's(T,T,T,H,H), which can arrange themselves in 5!/(3!)(2!)=10
case 2) kate wins in 4 draws and losses in the 1 draw. so we have 4 T's and 1H (T,T,T,T,H), which can arrange themselves in 5!/4!=5
thus total no. of favorable ways=10+5=15
and thus required probability=15/32