Statistics help

Question

Suppose you have 18 red marbles and 24 blue what is the probability of pulling two red at the same time ?

Answer 1

18R, 24B

p(R) = 18/(18+24) = 18/42 = 3/7

P(2nd R) = 17/41

P(2R) = 3/7 x 17/41 = 51/287= about 18% chance of two red

try a simple similar problem

3 marbles, 2R, 1B

draw 2

there's a chance of 2R or 1R1B

one way to get 2R, two ways to get 1R1B

p(2R)=1/3, p(1R1B) = 2/3

P(first R) = 2/3

P(2nd R) = 1/2

P(2R) = 2/3 x 1/2 = 1/3

multiply the probability of the 1st being Red, times the probability of the 2nd being Red to

get the probability of 2 Reds