Official Quant thread for CAT 2013

@sujamait said:
If 'a' is an odd natural number and 'b' is an even natural number, what is the total number of solutions of theequation ab = 2a + b + 598?A. 16B. 12C. 5D. 6E. 8
ab = 2a + b + 598
ab - 2a - b + 2 = 600
(a - 1)(b - 2) = 600 = p*q

a = p + 1 and b = 2 + q

So, p and q both are even, say 2n and 2m

mn = 150 = 2*3*25

So, m can take 2*2*3 = 12 values

Hence, 12 solutions
A coin is biased in such a manner that the chances of getting head is twice the chances of getting tail.Find the probability that the heads will appear an odd number of times in n tosses of the coin.
@19rsb said:
A coin is biased in such a manner that the chances of getting head is twice the chances of getting tail.Find the probability that the heads will appear an odd number of times in n tosses of the coin.
options?
@krum said:
options?
bhai dont hav d options and OA .......just seen dis question somewhere......thought it is a worth sharing question........with the hope of gaining some knowledge.....plz at least share ur thoughts on dis....
@19rsb said:
A coin is biased in such a manner that the chances of getting head is twice the chances of getting tail.Find the probability that the heads will appear an odd number of times in n tosses of the coin.
nC1*p^1*q^(n-1) + nC3*p^3*q^(n-3) + ....nCn*p^n*q^0 ---If n is odd

p= 1/3 , q =2/3 ??
@19rsb said:
A coin is biased in such a manner that the chances of getting head is twice the chances of getting tail.Find the probability that the heads will appear an odd number of times in n tosses of the coin.
Got the same equation as that of @ScareCrow28
Solving through options would have been easier
@sujamait said:
If 'a' is an odd natural number and 'b' is an even natural number, what is the total number of solutions of theequation ab = 2a + b + 598?A. 16B. 12C. 5D. 6E. 8 (2
ab = 2a + b + 598
=> (a-1)*(b-2) = 600 = 2^3*3*5^2

Here, a = odd and b = even
So, a-1 = even and b-2 = even

Hence, Both will have a factor of 2
Therefore 2^2 is common on both sides and can be striked out.
p*q = 2*3*5^2
No of factors = 12


@19rsb Nc1*(2/3)*(1/3)^(n-1) + Nc3*(2/3)^3*(1/3)^(n-3)......Nc(last odd number)*(2/3)^last odd number*((1/3))^(n- last odd number)
@ScareCrow28 said:
nC1*p^1*q^(n-1) + nC3*p^3*q^(n-3) + ....nCn*p^n*q^0 ---If n is oddp= 1/3 , q =2/3 ??
My interpretation of your solution
here you have taken n as odd
also q is prob of head and p is prob of tail
Now if n is odd and every time in q's power (odd- odd)=even.......is reflecting the probability of head appearing even times(in question it has been asked for odd times)
One more thing ,don't you think one more case, when n is even is also required(as n is number of tosses,it can be either even or odd)
PS :dont hav d OA ....so not sure about any of my claims(just interested in some hard core discussion)
@19rsb said:
My interpretation of your solutionhere you have taken n as oddalso q is prob of head and p is prob of tailNow if n is odd and every time in q's power (odd- odd)=even.......is reflecting the probability of head appearing even times(in question it has been asked for odd times)One more thing ,don't you think one more case, when n is even is also required(as n is number of tosses,it can be either even or odd)PS ont hav d OA ....so not sure about any of my claims(just interested in some hard core discussion)
Firstly, the question is the probability of finding odd no of heads in "n" tosses.
So, Suppose n=odd (I know it can be even, the equation won't be much different)
Then, no of heads can be = 0, 1, 2, 3, 4,. ....n..

No of heads odd = 1, 3, 5, .....n (If n=odd)
= 1, 3, 5, ...(n-1) --(If n= even)

So, No of ways of 1 head is = nC1*p^1*q^(n-1)
Similarly for others
Final equation would come out to be the one posted earlier.. :)
@ScareCrow28 said:
Firstly, the question is the probability of finding odd no of heads in "n" tosses.So, Suppose n=odd (I know it can be even, the equation won't be much different) Then, no of heads can be = 0, 1, 2, 3, 4,. ....n..No of heads odd = 1, 3, 5, .....n (If n=odd)= 1, 3, 5, ...(n-1) --(If n= even)So, No of ways of 1 head is = nC1*p^1*q^(n-1) Similarly for othersFinal equation would come out to be the one posted earlier..
bhai unable 2 get u completely.......(my inabilty)
but still ur method has shown me a direction of approaching these kind of questions
ab jab madad kr hi rhe ho toh.....yeh bhi dekh lo zara(Condition:No OA no options)
Six persons try to swim across a wide river.Its known that on an average only three persons out of 10 are successful in crossing the river.Whats the probability that at most four of the 6 persons will cross the river safely?
@19rsb said:
bhai unable 2 get u completely.......(my inabilty)but still ur method has shown me a direction of approaching these kind of questionsab jab madad kr hi rhe ho toh.....yeh bhi dekh lo zara(Condition:No OA no options)Six persons try to swim across a wide river.Its known that on an average only three persons out of 10 are successful in crossing the river.Whats the probability that at most four of the 6 persons will cross the river safely?
Working on same lines
P(at most 4 cross) = 1 - P(5 cross) - P(6 cross)
= 1 - [ 6C5*p^5*q^1 + 6C6*p^6]
= 1 - [ 3^5 * 45/10^6 ]
= 1 - 0.0109 = 0.989 ..
@ScareCrow28 said:
Working on same linesP(at most 4 cross) = 1 - P(5 cross) - P(6 cross)= 1 - [ 6C5*p^5*q^1 + 6C6*p^6] = 1 - [ 3^5 * 45/10^6 ]
Here p = 0.3 and q = 0.7 right?
We'll need a calculator to solve these kind of questions :(
@soumitrabengeri said:
Here p = 0.3 and q = 0.7 right?We'll need a calculator to solve these kind of questions
Yeah.. Ans is coming out to be 0.98 ..Itna zada?? posssible?
@ScareCrow28 said:
Yeah.. Ans is coming out to be 0.98 ..Itna zada?? posssible?
Exactly!! Even i tried just now..itna kaise aaya???
@19rsb I doubt the final answer 😞 Please confirm
@19rsb said:

Six persons try to swim across a wide river.Its known that on an average only three persons out of 10 are successful in crossing the river.Whats the probability that at most four of the 6 persons will cross the river safely?
P(crossing) = 3/10

P(at most 4 crosses) = 1 - [ P(5) + P(6) ] = 1 - [ 6C5 *(3/10)^5 *(7/10) + 6C6*(3/10)^6]

= 1 - [(3/10)^6 * (14 + 1)] = 1 - (0.3)^6*15 = 0.989
@ScareCrow28 said:
Yeah.. Ans is coming out to be 0.98 ..Itna zada?? posssible?
Why not. If out of 10, only 3 are most probable to cross, hence in case of 6, the most of the favorable cases are there in P(1 cross), P(2 cross) [Note the power of 7 in each]. So, it will be highly likely that at most 4 cross.
@YouMadFellow said:
Why not. If out of 10, only 3 are most probable to cross, hence in case of 6, the most of the favorable cases are there in P(1 cross), P(2 cross) [Note the power of 7 in each]. So, it will be highly likely that at most 4 cross.
Yeah..should be the right answer if you too got it 😛 I was just skeptical after getting such high probability
@ScareCrow28 said:
Yeah..should be the right answer if you too got it I was just skeptical after getting such high probability
Oh God, don't trust for calculations for me :splat: .. I have made mistakes as silly as 4*2 = 6 !

Find the value of C0 - C2 + C4 - C6 + C8................ using binomial theorem
C0,C2........ represents nC0, nC2........ (No OA, so show approach, well, you know what, one who is confident should always show approach no matter what 😐 )