9 A bag contains 9 discs numbered 1, 2, 3, 4, 5, 6, 7, 8, 9 .
- Andrea chooses 4 discs at random, without replacement, and places them in a row.
(a) How many different 4 -digit numbers can be made?
(b) How many different odd 4-digit numbers can be made? - Andrea's 4 discs are put back in the bag. Martin then chooses 4 discs at random, without replacement. Find the probability that
(a) the 4 digits include at least 3 odd digits,
(b) the 4 digits add up to 28 .