P(winning something) = 1 - P(winning nothing) = P(drawing a white marble)

There are 25 white marbles

and 25+20+10+5 = 60 marbles.

The probability of drawing a white marble is

25 ways out of 60 ways. In probability theory,

"out of" means "over", so the probability of

drawing a white mable is 25 over 60 or 

and that reduces to 

So 

P(winning something) = 1 - P(winning nothing) = 

1 - P(drawing a white marble) = 1 -  =  -  = .

So you'll average winning something 7/12ths of the 
times you purchase just one chance.  You'll win something 
7 times out of every 12 times you purchase just one chance.

Edwin