blitzkriegxX wrote:

There are 15 marbles in a jar consisting of only yellow and white marbles. The probability of choosing 2 white marbles, one after the other, is \(\frac{2}{21}\). How many yellow marbles are in the jar?

(A) 2

(B) 5

(C) 7

(D) 9

(E) 10

Where i went wrong, went too fast to solve this question.

Most important tip -> write in

what you want to calculate

To find -> Yellow marbles Y

Choosing 2 White Marbles, one after the other, will mean

n = number of white marbles

n /15 * (n-1)/14 = 2/21

n^2 - n - 20=0

n^2 - 5n + 4n - 20 =0

( n - 5 ) (n + 4 ) = 0

n =5

Number of Yellow marbles = 15 - n = 15 - 5 = 10

_________________

If you notice any discrepancy in my reasoning, please let me know. Lets improve together.

Quote which i can relate to.

Many of life's failures happen with people who do not realize how close they were to success when they gave up.