Karen has exactly 6 quarters, 5 dimes, and 10 nickels in her pocket. If she withdraws a number of these coins at random, how many coins would she have to withdraw to ensure that she has at least a 50 percent chance of withdrawing at least one quarter?
i will go with choice 2nd here..
suppose that she does not with draw quarter in 1st.. then probability of dime or nickel = 15/21 = 5/7 ~ 0.72
and probability of withdrawing quarter in 1st is = 0.28
if the 1st coin is dime or nickel then the probability of withdrawing quarter 2nd time is 6/20 = 3/10 = 0.3
together - 0.28 + 0.3 = 0.58 which is greater than 0.5 or 50% chances of withdrawing quarter..
so she should withdraw quarter in 1 of the 2 chances..