Math Group
Question:
pavelpimen
Student
High School - 10th Grade
There are 30 red and blue balls are in a box. How many balls are blue?
At a random drawing out, the ball extracted has the 7/15 probability to be red.
Answers:
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mcetner Teacher
High School - 12th GradeeNotes Editor
Posted by mcetner on Saturday February 7, 2009 at 9:33 AMSince you know the probability of drawing a red ball is 7/15, and you know the amount of balls in the box is 30, then you can make a proportion:
probability 7/15 = x/30. Solve for x by cross-multiplying and get x = 14. This means out of 30 balls, 14 are red.
Now, since the only other choice is blue, just subtract and 30 - 14 = 16 blue balls in the box.
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cburr Teacher
Middle SchooleNotes Editor
Posted by cburr on Saturday February 7, 2009 at 9:38 AMThe key to solving this problem quickly is to note the connection between 30 and 15 (30 = 15 times 2). But, even if you don't see that, you can figure it out with basic fraction math.
The problem tells you that if you pull out 15 balls, 7 of them will be red. But we have 30 balls, not 15. How many of the whole thing are red?
7 ?
__ = __
15 30
In the denominator you have to multiply 15 by 2 to get to 30, so do the same in the numerator -- 7 times 2 is 14.
7 x 2 14
__ = __
15 x 2 30
Remember, this is how many are red. Now we have to find out how many are blue.
30 - 14 = 16
So, the answer is that 16 balls are blue.
