A jar contains 3 red marbles and 2 blue marbles. If two marbles are selected at the same time, what is the probability that both marbles will be red?
How can I solve the above with the approach 1-Probability of None?
Thanks
Probability
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- knight247
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3 Red 2 Blue
For solving using the complementary method
Probability that both are red+Aleast 1 is blue=1
For atleast one being blue we could have the following scenarios
BB BR RB
(1)BB=2/5*1/4=1/10
(2)BR=2/5*3/4=3/10
(3)RB=Same as #2 so 3/10
Adding all three scenarios we have 3/10+3/10+1/10=7/10
Subtracting this from 1 we have 1-7/10=3/10
Direct method
RR
3/5*2/4=3/10
For solving using the complementary method
Probability that both are red+Aleast 1 is blue=1
For atleast one being blue we could have the following scenarios
BB BR RB
(1)BB=2/5*1/4=1/10
(2)BR=2/5*3/4=3/10
(3)RB=Same as #2 so 3/10
Adding all three scenarios we have 3/10+3/10+1/10=7/10
Subtracting this from 1 we have 1-7/10=3/10
Direct method
RR
3/5*2/4=3/10
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total marbles 5(3red+2blue)
selecting 2 from 5 is 5c2=10
for favorable now selecting 2 red from 3 is 3c2=3
probability =favorable/total=3/10
selecting 2 from 5 is 5c2=10
for favorable now selecting 2 red from 3 is 3c2=3
probability =favorable/total=3/10