a) (i) Is correct
(ii) This asks "The second disc is white if the first disc is green" not "The second disc is white AND the first disc is green" you don't need to include the chance of getting a wite disc first because it states that you did in fact get it first. Just find the chance of getting a green disc when there are 14 discs
b) (i) Only 2 of the discs are red and 1 disc is not red. If R is red and O is other, the disks could be picked in the orders RRO, ROR, ORR. Find the probability of each happening and add them up.
(ii) Is correct
In a lot of 2 you used 0.09 instead of 0.9 and 0.03 instead of 0.3
a) i) Your method is right
ii) You found the chance that both don't come the next day but that includes the chance of one coming and the other not.
iii) One or more letters arriving is equal to the chance of [neither arriving] Not happening. You know the chance of neither arriving from ii)
b) I'm not sure about this 1
3. a) Use 1/31 instead of 1/3
b) Same as a)
c) Find the chance that they are all on any given day, say... January 4th. There are 31 ways in which they can all be on the same day so multiply the chance of them all being on January 4th by 31.
d) You want to find the chance they are all on different days. There are 31 days which Shahid's birthday can be on; there are 30 days that Tracy's birthday can be on without it being on the same day as Shahid's. There are 29 days which Dwight's birthday can be on without it being on the same day as either of the others. Therefore there are 31*30*29 ways in which their birthdays can be different.
In total there are 31*31*31 ways you can choose their birthdays to be arranged.
Find the chance that their birthdays are different.