Posted by Mingma Sherpa on Thursday, May 5, 2011 at 6:48pm.
A sack contains yellow marbles and black marbles. If one marble is drawn at random, the probability that it is yellow is 7/8. Six yellow marbles are added to the sack. Now, if one marble is drawn, the probability that it is yellow is 9/10. How many yellow and black marbles were initially in the sack?
Algebra - Reiny, Thursday, May 5, 2011 at 7:30pm
yellow marbles -- y
black marbles --b
so y/(y+b) = 7/8
8y = 7y + 7b
y = 7b
yellow marbles -- y+6
black marbles --- b
(y+6)/(y+6+b) = 9/10
10y+60 = 9y+54+9b
y = 9b - 6
then 9b-6 = 7b
b = 3
then y = 7(3) or 21
so there were 21 yellow and 3 black
for a total of 24 marbles
Prob(yellow) = 21/24 = 7/8
27 yellow, still 3 black, total 30
prob(yellow) = 27/30 = 9/10
Algebra - Mingma Sherpa, Thursday, May 5, 2011 at 7:55pm
how did you get a positive 6
Algebra - Reiny, Thursday, May 5, 2011 at 9:28pm
In your question, didn't it say that 6 yellow marbles were added ????
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