There are 1000 sealed envelopes in a box, 10 of them contain a cash prize of Rs 100 each, 100 of them contain a cash prize of `Rs50 each and 200 of them contain a cash prize of `Rs10 each and rest do not contain any cash prize. If they are well shuffled and an envelope is picked up out, what is the probability that it contains no cash prize?
Total number of sealed envelopes in a box, n(S) = 1000
Number of envelopes containing cash prize = 10 + 100 + 200 = 310
Number of envelopes containing no cash prize,
n(E) = 1000 - 310 = 690
P(E) = 
⇒ P(E) = 0.69
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