Question

A box contains 500 envelopes of which 75 contain $100 in cash, 150 contain $25, and 275 contain $10. An envelope may be purchased for $25. What is the sample space for the different amounts of money? Assign probabilities to the sample points and then find the probability that the first envelope purchased contains less than $100

Answer 1

The first sentence pretty much is the sample space. 
p($100) = 75/500 = 3/20 
p($25) = 150/500 = 3/10 
p($10) = 275/500 = 11/20 
p(< $100) = 3/10 + 11/20 = 17/20