math
posted by A on .
A jar contains four blues marbles and two green marbles. Without looking, two marbles are drawn from the jar. What is the probability that two marbles with the same color will be selected?

If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.
For blue = 4/6 * 3/5 = 2/5
For green = 2/6 * 1/5 = 1/10
Eitheror probabilities are found by adding the individual probabilities.
2/5 + 1/10 = ? 
2/5+1/10 is 5/10.
But in my book it states that the answer is 7/15, without an explanation. 
Either two blues or two greens
prob of 2 blues = (4/6)(3/5) = 2/5
prob of 2 grees = (2/6)(1/5) = 1/15 , this is where PsyDAG slipped up
so prob of 2 of same colour = 2/5 + 1/15 = 7/15 
⁄Mia Culpa! Sorry.