21.A buys 3 tickets of a lottery in which total number of tickets is 12 out 3 carry prizes on them. A is a chance of getting prize, is

22.Seven white balls and three black balls are randomly placed in a row. The probability that no two black balls are placed adjacently, equals

23.A car is parked by an owner amongst 25 cars in a row, not at either end. On his return he finds that exactly 15 places are still occupied. The prohability that both the neithbouring places are empty is

24.A man alternately tosses a coin and throws a die. The probability of his getting a head on the coin before he gets 4 on the die is

25.A and B throw a die. The probability that A throw is not greater than B is