A bag contains 4 red marbles and 5 blue marbles. If two marbles are chosen at random, one at a time and without replacement, what is the probability of getting two red marbles?

1 Answer
Dec 19, 2015

#P#(2 red marbles)# = 1/6#

Explanation:

#P#(2 red marbles)# = P(#first marble is red#)xxP(#second marble is red, given first is red#)#

Since there are initially #4# red marbles in a collection of #4+5 =9# marbles
#color(white)("XX") P#(first marble is red)#=4/9#

If the first marble is red then there are 3 red marbles left in a collection of #9-1=8# marbles
#color(white)("XX")P#(second marble is red, given first is red)#=3/8#

Therefore
#color(white)("XX")P#(2 red marbles)# =4/9xx3/8=12/72 = 1/6#