happy1992 wrote:
Thirty airmail and 40 ordinary envelops are the only envelop in a bag. Thirty-five envelops in the bag are unstamped, and 5/7 of the unstamped envelops are airmail. What is the probability that an envelope picked randomly from the bag is an unstamped airmail envelope?
(A) 1/7
(B) 1/3
(C) 13/35
(D) 17/38
(E) 23/70
Here we have 30 airmail and 40 ordinary envelops. Hence a total of 70 envelops.
Now 35 envelops are un-stamped a 25 of them are airmail. Hence the new distribution is :
1. Unstamped airmail 25
2. Stamped airmail 5
3. Unstamped normal envelop 10
4. Stamped normal envelop 30
Hence probability of unstamped airmail envelope should be 25/70.
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