What is the sum of the first four terms of sequence S?

Question

What is the sum of the first four terms of sequence S ?

(1) After the first two terms of S, the value of each term of S is equal to the average (arithmetic mean) of the last two preceding terms.

(2) The average (arithmetic mean) of the first three terms of S is 10.

OG Q 2017 New Question (Book Question: 219)

Nick90 · Accepted Answer

E

Assume 1st term a nd 2nd term b.

From 1st statement, 3rd term (c) = (a+b)/2

4th term = (b +c)/2

From 2nd statement, (a+b+c)/3 =10
a+b+ (a+b)/2 =30

a+b = 20 
Therefore, c=10

4th term = (b+c)/2

( we find out the sum of a and b but we can't say anything about individual values. Since, a can be 1,2.....so on and correspondingly b can be 19,18....so on)

So the answer is E

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MikeScarn · Answer

(1)  We could make S anything.

Not Sufficient.

(2)  Plug in numbers.

9, 10, 11, 100. Satisfies statement and sum is 130
9, 10, 11, 12. Satisfies statement and sum is 42.

Not Sufficient.

(1) + (2)  Plug in numbers.

9, 11, 10, 10.5. Satisfies both statments and sum is 40.5

5, 15, 10, 12.5. Satisfies both statements and sum is 42.5

Not Sufficient.

Answer:  E

09173140521 · Answer

WHAT IS THE BEST APPROACH TO SOLVE SUCH PROBLEM ?

Hanzel101 · Answer

According to statement 1 = x + y + (x+y)/2 +  /2 = S

No info about Sum, NS.

St 2- x+y+z = 30.

NS.

On combining,

x + y + (x+y)/2 = 30 => x+y = 20.

We can find the 3rd term with this info but since we have no idea what type of sequence it is w.r.t first three terms, we cannot determine x & y.

FYI, 3rd term will be (x+y)/2 = 10. (replacing x+y with 20). But, can you find x & y ? (x+y =20 ...x & y could be multiple values)

twistedphoton · Answer

What is the sum of the first four terms of sequence S?

(1) After the first two terms of S,the value of each term of S is equal to the average (arithmetic mean) of the last two preceding terms.

(2) The average (arithmetic mean) of the first three terms of S is 10.

Let first 2 terms be a,b
As per statement 1, sequence is: a, b, (a+b)/2, b +(a+b)/2 = a, b, (a+b)/2, (a+3b)/2
Sum = (4a + 6b)/2 (Sum can be anything, since we don't know values of a and b, so eliminate option A and D)

Let first 3 terms be a,b,c 
As per statement 2, a+b+c = 3*10 = 30
Now, 4th term can be any term, thus eliminate option B

Combining both statements:
a + b + (a+b)/2 = 30
a+b = 20
Sum = (4a+6b)/2 =   / 2
Sum = 2(a+b) + b
Sum = 2*20 + b = 40 + b
Since, we don't know the value of b, still not sufficient, Select Option E

ScottTargetTestPrep · Answer

Solution:

Question Stem Analysis:

We need to determine the sum of the first four terms of sequence S. If we can determine the values of the four terms, then we can determine their sum.
 
 Statement One Alone:

Since we don’t know the value of any of the 4 terms, statement one is not sufficient.

Statement Two Alone:

Even though we don’t know the value of any of the first 3 terms, we know their sum is 30. However, since we don’t know the 4th term, statement two is not sufficient.

Statements One and Two Together:

We can let a and b be the first two terms, then using the first statement, the 3rd term is (a + b)/2 = a/2 + b/2, and the 4th term is (b + (a + b)/2)/2 = b/2 + a/4 + b/4 = a/4 + 3b/4. From the second statement, we can create the equation:

a + b + a/2 + b/2 = 30

3a/2 + 3b/2 = 30

3a + 3b = 60

a + b = 20

However, since we don’t know the individual values of a and b, we can’t determine the 4th term. For example, if a = 8 and b = 12, then the 4th term is a/4 + 3b/4 = 2 + 9 = 11 and the sum of the first 4 terms is 30 + 11 = 41 (recall that the sum of the first 3 terms is 30). However, if a = 12 and b = 8, then then the 4th term is a/4 + 3b/4 = 3 + 6 = 9 and the sum of the first 4 terms is 30 + 9 = 39. Both statements together are not sufficient.

Answer: E

Fish181 · Answer

Can anyone try this non algebraically? I'm a little confused why it isn't C.

GMATBusters · Answer

Why do you think it is C ?

Provide your logic so that I can provide ethe flaw in your logic.

GMATBusters · Answer

St 1)

Let's denote the first two terms as x and y.

According to the given information, the third term is calculated as ((x + y)/2),

The fourth term is then ((y + (x + y)/2)/2).

Now, since the value of x and y can change, so the sum of first four will also change.

Not Suff.

St2)

Let's denote the first three terms as x, y and z

Sum of x, y and z = 30

Since there is no info about the fourth term, the sum of the first 4 terms cannot be found.

Not Suff.

Let's combine St1 and St2

Considering first 2 terms as x and y , we can form the following equation:

x + y + x/2 + y/2 = 30

or, x + y = 20

this will only gives that the third term is 20/2 = 10. and the sum of first 2 term is 20. It is not possible to find the 4th term unless we know the value of 2nd term as fourth term is the avg of 2nd and 3rd term .

Hence, even after combining, it is NOT possible to find the sum of first 4 terms.

Not Suff

Answer E

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What is the sum of the first four terms of sequence S?

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