Algebra
posted by Mingma Sherpa .
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?

original bag
yellow marbles  y
black marbles b
so y/(y+b) = 7/8
8y = 7y + 7b
y = 7b
new bag:
yellow marbles  y+6
black marbles  b
(y+6)/(y+6+b) = 9/10
10y+60 = 9y+54+9b
y = 9b  6
then 9b6 = 7b
2b=6
b = 3
then y = 7(3) or 21
so there were 21 yellow and 3 black
for a total of 24 marbles
check:
Prob(yellow) = 21/24 = 7/8
new bag:
27 yellow, still 3 black, total 30
prob(yellow) = 27/30 = 9/10 
how did you get a positive 6

In your question, didn't it say that 6 yellow marbles were added ????
Respond to this Question
Similar Questions

prealgebra
a bag contains 6 red marbles, 5 yellow marbles, 7 blue marbles, and 4 white marbles. if one marble is drawn at random from the bag, what is the probability that it will be white? 
MathProbability
Hello, I'm trying to figure out this probability problem. Can anyone help walk me through? 
Statistics
1.In a jar, suppose there are 3 blue marbles, 2 yellow marbles and one red marble. c. Suppose that 3 marbles are drawn from the jar without replacement. What is the probability that the 3 marbles are all different colors? 
algebra
a bag contains 3 green marbles, 2 yellow marbles, and 5 blue marbles. a marble is picked at random and replaced. a second marble is picked at random. what is the probability that the first marble is yellow and the second is not yellow. 
Math
There are 150 green marbles and 100 yellow marbles in a bag that contains 250 marbles. If only yellow marbles are added to the bag so that the probability of randomly drawing a yellow marble from the bag becomes 3/5, how many yellow … 
Math
Bobby has a bag of marbles with nine red marbles, 13 yellow marbles and six orange marbles. He picks one marble and keeps it out. He wants to find the probability of picking a yellow marble by randomly picking one more marble from … 
algebra 1
The bag contains three red marbles, two blue marbles, and seven yellow marbles. Two marbles are randomly drawn from the bag. What is the probability of drawing a blue marble, replacing it, and then drawing a yellow marble 
maths
99. From a box containing 5 black marbles,8 pink marbles, 6 white marbles, and 5 yellow marbles, Kim removed 4 marbles,one of which was black.If she removes one more marble at random,what is the probability that it will be black? 
math
A bag of marbles contains 8 red marbles and 6 yellow marbles. Two marbles are drawn out of the bag at random without replacement. What is the probability that first a red marble then a yellow marble are drawn? 
Is my answer correct tutors
A bag of 7 blue marbles, 8 green marbles, 9 yellow marbles. If one marble is drawn from the bag but not replaced, what is the probability of drawing a blue marble then a yellow marble?