500 tickets are sold for a prize draw. the probability that bill wins first prize is 1/20. how many tickets did he buy?

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!