A for effort! Math is (NOT) for me.

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Sometime ago, in one of newsletters I came across a question to find next number in series. The series was 3, 11, 20, 27, 29, 23. The next number in series needed to be found. Well, I tried as much as I can and stopped when my brain gave up on me. I was never too bright as math. I wasn’t too bad but I wasn’t this old either at that time.

So can you help me find answer?

A for effort was given to a colleague who answer this correctly and I can clearly see why. He answered it very thoroughly and I am going to publish it with due credits to him (name not published for privacy).

As he wrote:

“The series given was 3, 11, 20, 27, 29, 23

The solution, the way I saw it was to find some function, f(x) that would equal:

f(1) = 3, f(2) = 11, f(3) = 20, etc

so that I could predict the value of f(7).

I found the differences between each set of answers to go down 3 levels, with a shared difference of -3.  Three levels means I need to find a third degree polynomial  f(x) = ax^3 + bx^2 + cx + d.

I then needed to find the values for a, b, c and d.  There are 4 unknowns, so I need 4 equations.

The four equations are:

f(1) = a(1)^3 + b(1)^2 + c(1) + d = 3

f(2) = a(2)^3 + b(2)^2 + c(2) + d = 11

f(3) = a(3)^3 + b(3)^2 + c(3) + d = 20

f(4) = a(4)^3 + b(4)^2 + c(4) + d = 27

Using linear algebra and putting these into a matrix and reducing to echelon form, I solved for a, b, c and d.

a=-1/2, b=7/2, c=1, d=-1

That makes f(x) = -1/2 x^3 + 7/2 x^2 + x – 1

Calculating out f(x):

X

-1/2 x^3

7/2 x^2

X

-1

f(x)

0

0

0

0

-1

-1

1

-0.5

3.5

1

-1

3

2

-4

14

2

-1

11

3

-13.5

31.5

3

-1

20

4

-32

56

4

-1

27

5

-62.5

87.5

5

-1

29

6

-108

126

6

-1

23

7

-171.5

171.5

7

-1

6

8

-256

224

8

-1

-25

9

-364.5

283.5

9

-1

-73

Finally, the answer to the question is 6, highlighted above in red.”

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