Question 979969
Q> There is a stack of folders. the probability of drawing out a yellow folder from the stack is 7/12. after 18 yellow folders are taken out, the probability of drawing out a yellow folder now is 4/9. how many yellow folders are there initially? 


A> Let us consider there are total 12x folders out of which 7x are yellow.


Now, 18 yellow folders are taken out; remaining yellow folders= 7x-18.
Remaining total folders= 12x-18


Now the probability of a yellow folder is given as (7x-18)/(12x-18).


Thus, (7x-18)/(12x-18) = 4/9

=> 63x-162=48x - 72
=>15x=90
=> x= 6.

Initially there are 7x or 42 folders.


Thanks,
PRD
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