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Statistics- probability help

This question is in my homework but I have no idea how to even start answering the questions in part B, please help!

2. A bag contains 9 discs numbers 1, 2, 3, 4, 5, 6, 7, 8, 9.
(a) Andrew choses 4 discs at random, without replacement, and places them in a row.
(i) How many different 4-digit numbers can be made?
(ii) How many different even 4-digit numbers can be made?

(b) Andrew’s 4 discs are put back in the bag. Claire then choses 4 discs at random, without replacement.
Find the probability that:
(i) The 4 digits include at least 3 odd digits.
(ii) The 4 digits add up to 28.

Reply 1

ghostwalker

Original post by leele

(b) Andrew’s 4 discs are put back in the bag. Claire then choses 4 discs at random, without replacement.
Find the probability that:
(i) The 4 digits include at least 3 odd digits.

Break it down into two cases:
Number of ways of getting 3 odd numbers.
Number of ways of getting 4 odd numbers.

Add, and divide by the total number of possibilities.

(ii) The 4 digits add up to 28.

Be systematic. List the digits in descending order.

Assume there is a 9, and an 8 in the sum, then what are the other possibilites.
Replace the 8 with a 7, so there are 9 and 7 in the sum - what are the other possibilities.
Etc.
When you've exhausted 9, assume the two highest numbers are 8 and 7, etc.

There are, I think only two possibilities in all.