Form 4/5 Probability Question Types

Just a quick recap:

Form 4:

1. Sample Space

2. Event

3. Probability without unknown

a) 2 out of 10 girls are fat = 1/5

b) 2 out of 10 are fat, 100 girls now, how many fat = 1/5 x 100

c) 2 out of 10 are fat, now got 20 fat, how many girls total = 20 divide 1/5

4. Probability with unknown

a) 2 out of 10 are fat, now got 20 fat, how many girls total = 20/T = 1/5 (Then simultaneous) (same as 3c. can fit both category)

b) I got 5 red ball, 3 blue ball and dunno how many green ball.

(i) If probability of finding red ball is 1/2. Find no. of green ball or probability of green ball.

(ii) If they tell me probability of green ball is 1/5, find how many green ball are there, or how many balls in total.

Form 5:

1. One set

a) I got 10 bf and 20 gf. If I pick two at random, find probability that second one is gf.

Explanation: So first one becomes not important. I only focus on second one.

b) I got 10 bf, 20 gf and 30 agua. If I pick one at random, find probability that I pick either guy or girl.

Explanation: guy or girl means, guy, OR girl only ma, either one come out will be fine to me. This one you ADD probability. As long see the word "OR", you add.

2. Two sets with constant total

a) I got 5 cards, ABCDE, pick one, put back the card, then pick another one. Find probability that two cards are A.

Explanation: First attempt, 5 choose 1, second attempt, still 5 choose one. Probability is 1/5 multiply 1/5. See the word "AND" means multiply. (two cards are A = A and A)

STUDENTS ALWAYS THINK ITS ONE SET. You pick two times, so you got 2 sets.

b) I got 2 bags, each got 5 balls, labelled 1, 2, 3, 4, 5. I pick one from each bag. Find probability that both are same number. 1/5 x 1/5 x 5.

Explanation: Probability of (1, 1) = 1/5 x 1/5. But (2, 2) also "both same number". so you can have also (3, 3), (4, 4), (5, 5), so I times 5. (See the word both are also use multiply. Cause both = and)

3. Two sets with decreasing total (hardest)

a) I got 5 cards, ABCDE, pick two. Find probability that I got one A, one B. 2 x 1/5 x 1/4

Explanation: I can either get A,B or B,A. Probability of getting A at first is 1/5, then choose B is 1/4 dy. Cause one card take out dy ma. Same goes to B, A. Thats why multiply two. Here some ppl might think its one set, cause I either get AB or BA. (Then they will do "1/5 x 1/4" + "1/5 x 1/4") Tell them, main focus here is decreasing total. at first total is 5, then become 4 dy. If lets say the example at 2b) I can have 1,1 OR 2,2 OR 3,3......if got so many SAME probability, why we so stupid go add? can multiply in fact. So tell them one set is not very appropriate.

b) I got 5 cards, ABCDE, pick two. Find probability that I got one A, then one B. 1/5 x 1/4

Explanation: this one can only A,B, cannot B,A. So dont multiply 2.

c) Edison Chen has 9 Indian wives and 5 Bangladesh wives. If he has to pick 2 wives every night, find the probability that both are from the same race. 9/14 x 8/13 + 5/14 x 4/14

Explanation: I can get Indian Indian or Bangladesh Bangladesh. After pick one Indian, left 13 ppl and 8 indian, same goes to Bangladesh. So we (I x I + B x B) in this case they think one set or two sets with decreasing total oso give them correct. Cause II or BB, but cannot do by 2 x P(II) like that.

d) Edison Chen has 9 Indian wives and 5 Bangladesh wives. If he has to pick 2 wives every night, find the probability that at least one is Indian. IB or BI or II.

Explanation: You pandai pandai know la...I sangat lazy dy.

basically is like that la...Form 5 one actually one question can fit many groups. cause' u know teachers like to do rojak campur everything. So some questions can fit 2 groups, depends on how ur students interpret. so everyone...ADD OIL for the GAMES!!!!!!