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A group of friends have some money which was in an increasing GP. The total money with the first and the last friend was Rs. 66 and the product of the money that the second friend has and that the last but one friend has was Rs. 128. If the total money with all of them together was Rs. 126, How many friends were there?
A) 6
B) 5
C) 3
D) Cannot be determined

Answer
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Hint:
Here we will consider an increasing GP a,ar,ar2,.......arn1. Then from the given question we will try to form equations. We will have 2 equations. Then we will put the value of equation 2 in equation 1. Then we will get the value of ‘a’. Let’s see how we can solve it.

Complete step by step solution:
Let’s consider the GP a1,a2,a3,......an
We know that an=arn1
Putting the value n = 1 in the above formula;
a1=ar11
a1=a
Similarly, we can say that the increasing GP will be a,ar,ar2,.......arn1.
Sum of 1st term and the last term is a + arn1= 66 a(1+rn1)=66……. (1)

Product of second and last second term = (ar)(arn2)=128
= a2rn2+1=128
= a2rn1=128
= rn1=128a2……. (2)

Putting the value rn1=128a2in equation (1), we get;
a(1+rn1)=66
a(1+128a2)=66
Solving the equation for a; we get
a2+128=66aa266a+128=0
Factoring the above quadratic equation, we get;
a264a2a+128=0a(a64)2(a64)=0
(a – 64) (a – 2) = 0
a = 64 and a = 2

It is not possible to have 2 first terms. So, we will put the values of a in
rn1=128a2
Putting the value, of a = 64
rn1=128(64)2=132
rn=132r
Now, Putting the value, a = 2
rn1=128a2
rn1=128(2)2=1282×2=32
rn1=32………… (3)
rn.r1=32rn=32r
Clearly it is mentioned in the question that we have increasing GP. Thus, in increasing GP we cannot consider the value of rn which is less than 1 as it will not be an increasing GP anymore. So, we cannot take rn = 132r. That is why we would consider a = 2 and rn= 32r
Sum of total money they have Sn = 126
In GP, Sn=a(rn1)r1
Sn= 2(32r1)r1=126
Solving for r;
32r – 1 = 63(r – 1)
32r – 1 = 63r – 63
31r – 62 = 0
r = 6231=2
Now we know that from above that rn1=32
Putting the value of r = 2;
rn1=32
2n1=25
Base is same, so we can equate their powers;
n1=5n=6
Thus, the total number of friends is 6.

Hence, Option A is the correct option.

Note:
In the above question, first you need to try to find out the value of a &rn. You have to keep in mind that you have to find a number of friends. It means you need to find the value of ‘n’. It is given thatSn=a(rn1)r1= 126. Now, you have to put the value of a & rnin this equation. After solving this you can find the value of ‘r’. Now, you can put the value of ‘r’ in equation 3. You will finally get the value of ‘n’.