Question

500 tickets are sold for a prize draw. The probability that Bill wins first prize is 1/20. How many tickets did he buy?

Answer 1

He bought 1 ticket, for the first prize only 1 of those 20 tickets is Bill's

Answer 2

We know, Probability = Favorable outcome / Total Outcome
Here, P = 1/20
Total Outcome = 500

Substitute their values, 
1/20 = x / 500
x = 500/20
x = 25

In short, Your Answer would be 25 Tickets

Hope this helps!