Statistics and Probability

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 Rs 50 each and 200 of them contain a cash prize of Rs 10 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?