Exactly one tenth of the students at Dave’s high school are male. Exactly
one third of all seniors at his school are male. Let p be the probability
that a male at his school is a senior and let q be the probability that a
female at his school is a senior. What is the value of p/q?
(a) 4
(b) 4.25
(c) 4.5
(d) 4.75
(e) 5
Probability
This topic has expert replies
-
- Legendary Member
- Posts: 1035
- Joined: Wed Aug 27, 2008 10:56 pm
- Thanked: 104 times
- Followed by:1 members
let there be 100 students. 10 of them are male=>90 of them female
let x be the number of seniors. 100-x non seniors
male+senior: 1/3x; female +senior: 2/3x
p: ( a male who is a senior)=(1/3x)/10
q: (a female who is a senior)=(2/3x)/90
p/q=(1/3)*(3/2)*(1/10)*90=9/2=4.5
hence, C
let x be the number of seniors. 100-x non seniors
male+senior: 1/3x; female +senior: 2/3x
p: ( a male who is a senior)=(1/3x)/10
q: (a female who is a senior)=(2/3x)/90
p/q=(1/3)*(3/2)*(1/10)*90=9/2=4.5
hence, C
Cool Scooby. Or for the formula wonks:scoobydooby wrote:let there be 100 students. 10 of them are male=>90 of them female
let x be the number of seniors. 100-x non seniors
male+senior: 1/3x; female +senior: 2/3x
p: ( a male who is a senior)=(1/3x)/10
q: (a female who is a senior)=(2/3x)/90
p/q=(1/3)*(3/2)*(1/10)*90=9/2=4.5
hence, C
P=P(S|M)= P(S and M)/P(M) = 1/3/1/10 =10/3
Q=P(S|F)= P(S and F)/P(F) =2/3/9/10 = 20/27
P/Q= 10/3 x 27/20 =4.5
-
- Junior | Next Rank: 30 Posts
- Posts: 26
- Joined: Thu Dec 11, 2008 12:02 am
- Thanked: 1 times
dtweah wrote:
Cool Scooby. Or for the formula wonks:
P=P(S|M)= P(S and M)/P(M) = 1/3/1/10 =10/3
Q=P(S|F)= P(S and F)/P(F) =2/3/9/10 = 20/27
P/Q= 10/3 x 27/20 =4.5
Actually, P(S and M) is 1/3 / P(S) and P(S and F) is 2/3 / P(S), but the P(S) is simplified in P/Q. I suppose you know that, but just in case.
- dumb.doofus
- Master | Next Rank: 500 Posts
- Posts: 435
- Joined: Sat Sep 27, 2008 2:02 pm
- Location: San Jose, CA
- Thanked: 43 times
- Followed by:1 members
- GMAT Score:720
I am not sure this is the right way to do it.. coz you are getting probability greater than 1 i.e. P and Q both of them are greater than 1.. I thought probability was always between 0 and 1...dtweah wrote:Cool Scooby. Or for the formula wonks:scoobydooby wrote:let there be 100 students. 10 of them are male=>90 of them female
let x be the number of seniors. 100-x non seniors
male+senior: 1/3x; female +senior: 2/3x
p: ( a male who is a senior)=(1/3x)/10
q: (a female who is a senior)=(2/3x)/90
p/q=(1/3)*(3/2)*(1/10)*90=9/2=4.5
hence, C
P=P(S|M)= P(S and M)/P(M) = 1/3/1/10 =10/3
Q=P(S|F)= P(S and F)/P(F) =2/3/9/10 = 20/27
P/Q= 10/3 x 27/20 =4.5
So again i am really not sure.. May be you are right.. but then I dont know..
Just another approach: I didnt get this approach in the first two mins.. so agreed I would have had to pass this question if I got this in the exam..
Let the total number of males be M
Let the total number of females be F
Let the total number of Senior males be Ms
Let the total number of Senior females be Fs
According to the question:
1/10(M + F) = M (Exactly one tenth of the students at Dave’s high school are male) --------------- (1)
1/3(Ms + Fs) = Ms (Exactly one third of all seniors at his school are male) ---------------- (2)
Now P = Ms/M and Q = Fs/F
P/Q = MsF/FsM ----------- (3)
From (1) F/M = 9 and from (2) Ms/Fs = 1/2
Putting these values in (3) gives 9/2 or 4.5
so P/Q = 4.5
One love, one blood, one life. You got to do what you should.
https://dreambigdreamhigh.blocked/
https://gmattoughies.blocked/
https://dreambigdreamhigh.blocked/
https://gmattoughies.blocked/