Easy probability from official downloadable test

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Easy probability from official downloadable test

by sev » Mon Jun 04, 2007 3:16 pm
If a flower bed consists of 30 roses, each of which is either white or red. If a rose is to be randomly selected from the flower bed, the probability that it will be white is twice the probability that it will be red, how many white roses are in the flower arrangement?


I guessed 25, because i was running out of time but I cant seem to solve this one, even though it's easy

i know the probability that the white is going to be choses is twice the probability of the red being selected.

How would this be solved?
Source: — Problem Solving |

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Long winded way...

Let probability of red = x. White = 2x (given)
We know x + 2x = 1 (since there are only 2 kinds of roses)
So, x = 1/3.
Probability of white = 2x = 2*1/3 = w / 30
So, w = 20

Short way...

Ratio of white to red = 2:1, so number of white = 2/3 * 30 = 20