Good morning/evening everyone. Dad here, helping daughter navigate higher level maths leaving cert book riddled with mistakes. There is one roughly in every subchapter up to the point where i am now. I was hoping really hard that the Probability chapter doesn't have any, so I am hoping I am wrong instead of the book.
Question says: A bag contains 5 red and 4 green discs, identical in all but colour.
Three disks are drawn at random from the bag without replacement.
Find the probability that
(i) they are all the same colour
(ii) at least one is red
(iii) at most one is green
My results:
(i) same colour means 5:9x4:8x3:7 + 4:9x3:8x2:7 = 1:6 same as the book
(ii) at last one is red means either one or two or three red, so 5:9x4:8x3:7 + 5:9x4:8x4:7 + 5:9x4:8x3:7 = 25:63. The answer in the book is 20:21. i chose three red plus two red one green plus one red two green. what am i doing wrong?
(iii) at most one is green is either one or none, so 4:9x5:8x4:7 + 5:9x4:8x3:7 = 5:18. The answer at the back of the book says 25:42. What am i doing wrong?
As i said, the book is full of mistakes but i do perform very badly too, depending which time of the day I'm doing this and i been grapling with this in my head since 2:30am until now 4:46.
Please help. Thank you in advance
Question says: A bag contains 5 red and 4 green discs, identical in all but colour.
Three disks are drawn at random from the bag without replacement.
Find the probability that
(i) they are all the same colour
(ii) at least one is red
(iii) at most one is green
My results:
(i) same colour means 5:9x4:8x3:7 + 4:9x3:8x2:7 = 1:6 same as the book
(ii) at last one is red means either one or two or three red, so 5:9x4:8x3:7 + 5:9x4:8x4:7 + 5:9x4:8x3:7 = 25:63. The answer in the book is 20:21. i chose three red plus two red one green plus one red two green. what am i doing wrong?
(iii) at most one is green is either one or none, so 4:9x5:8x4:7 + 5:9x4:8x3:7 = 5:18. The answer at the back of the book says 25:42. What am i doing wrong?
As i said, the book is full of mistakes but i do perform very badly too, depending which time of the day I'm doing this and i been grapling with this in my head since 2:30am until now 4:46.
Please help. Thank you in advance