# Trading Interview Question..Sequence & Series

##### New Member
81 , 90 , 83 , 94 , 82 , 108 , ?
Options are 96 , 98 , 100 , 102

98.

#### C S

##### Well-Known Member
This answer will go well with some people and not so well with others.

"98."

"I noticed every other number has the digit 8."

"I'm looking for a mathematical explanation, using knowledge of prime numbers and the zeta function --"

"Are you kidding me? I thought this was a trading interview. Trading isn't rocket science. It's about seeing opportunities and seizing them before others. I could spend another 5 minutes figuring your puzzle out, but what would that prove? A child could have reasoned the way I did and have gotten the answer. And that covers 99% of the situations that occur in the real markets. Maybe I got your puzzle wrong, but I'd win most of the time in the markets. "

##### New Member
not correct buddy...the logic is that we have to look into the series 81 , 83 , 82 , ?
as in it has 2 series in one sequence...bcuz i have done other questions of similar level..but this one cudn't..in all
others..there was some inherent logic..so yeah..if u cud figure out smthing solid..lemme know..its really bugging me ..thanks anyways for ur help..

#### Zerocool

##### Member
Out of curiosity is the correct answer 102?

If so, here is how I did it:

From 81 to 90, 9+
From 83 to 94, 11+
From 82 to 108, 26+

Taking the difference, 9-11 = 2 and 11-26 = 15
Multiplying them gives us 2 x 15 = 30
----------------------
Other side now
From 90 to 83, 7-
From 94 to 82, 12-

Taking the differences (in absolute values and I will explain later why), 7-12 = 5 and 12-XXX=YYY
------------------------
Therefore 5 x YYY = 30 -> YYY=6

I choose to take absolute numbers so I don't have an ANSWER which is greater than 108 due to the fact that in our series until now always the 3rd,5th etc item is smaller then the preceding one (2nd,4th etc)..

I hope the above make sense

Last edited:

#### euroazn

##### Well-Known Member
Your answer makes sense, but I don't like it, because you're extrapolating the pattern by complete guesswork... it's not a pattern "found" within the numbers given.

#### dray

##### New Member
81 , 90 , 83 , 94 , 82 , 108 , ?
Options are 96 , 98 , 100 , 102

9*9, 9*10, 9*9+2, 9*10+ 4, 9*9+1, 9*12, __?_

#### mutz

##### Member
C++

Started:
81, 81+90 = 171, 171 + 83 = 254, 254+ 94 = 348, 348+ 82 = 430, 430 +108 = 538

081
171
254
348
430
538
XXX

Isolate the hundreds and tens digits and take the first difference
08 > 17 > 25 > 34 > 43 > 53 > XXXX
9 8 9 9 10
-1 1 0 1 *second difference
apply fibonanci
-1, 1,0,1,1
for second difference to be one first difference =11 >> 53 + 11 = 64

making hundreds and tens digits have to be 64 only way to get there is 538 + 102 = 640

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