Probability as a Fraction
posted by Jeffrey .
In a bag there are 7 red tiles, 3 blue tiles, and 2 purple tiles. How many and what color tiles would you have to add to the bag so that the probability of picking red is 1/3?

you want red to be 1/3, you have 7 to choose from, so you want probability to be 7/21. YOu have 12, how many more non red tiles would you add to make 21?

12 in bag now
7/12 are red which is more than 1/3
Say we add x blue tiles
then
7/(12+x) = 1/3
12 + x = 21
x = 9
so add 9 blue tiles for example
check
7 red
12 blue
2 purple
that is 21 total
7/21 = 1/3 check