## What is the First Number of the Sequence?

If a number in a numeric sequence is 17 and the second number is 3, what is the first number?

I hesitated to add this because it’s poorly worded, ambiguous and the answer could be almost anything. I prefer teasers with a single answer, but there you go.

If you came up with a different answer and can explain how you did it, don’t think you’re wrong. It’s probably just as valid. Feel free to share yours in the comments.

My answer for the first number is 2.

Here’s how I got it.

The generic rule for a number in the sequence is: 2^(n – 1) + 1, where n is the position in the sequence.

Note: The teaser doesn’t specify the position of 17. In this case, it’s fifth.

Position 1: (so n = 1) is 2^(1 – 1) + 1 = 2

Position 2: 2^(2 – 1) + 1 = 3

Position 3: 2^(3 – 1) + 1 = 5

Position 4: 2^(4 – 1) + 1 = 9

Position 5: 2^(5 – 1) + 1 = 17

For the curious, the next 5 numbers of the sequence would be:

Position 6: 2^(6 – 1) + 1 = 33

Position 7: 2^(7 – 1) + 1 = 65

Position 8: 2^(8 – 1) + 1 = 129

Position 9: 2^(9 – 1) + 1 = 257

Position 10: 2^(10 – 1) + 1 = 513

Posted in Brain Teasers

30 Comments on "What is the First Number of the Sequence?"

SILVESTER says
June 24, 2015 @ 08:01

If The Number Of The Sequence Is 17 And 3 Third Number ,What Is The First Number

I found that version too ambiguous, so I reworded it slightly to make it solvable. No one has yet to explain what “the number of the sequence” is, so I made it, “a number in a numeric sequence”, which makes sense.

liliane says
June 30, 2015 @ 17:10

1

Charles says
July 1, 2015 @ 05:10

I agree with Liliane ans is 1

What’s the reasoning?

Gonzaga says
July 3, 2015 @ 02:42

that question is so complex the number of the squanse is 17 the next is 3 what is the first number for that u need to go back in A-level no other opt

Andrea says
July 18, 2015 @ 23:43

I also thought 1. It’s simply the sequence of odd numbers.

Hillary Pro says
July 20, 2015 @ 17:23

Me i think the first number is 1 not so:

Muhondogwa Jeremiah says
July 30, 2015 @ 05:04

thats the true setting of the question .but 1 is not the answer

kenny says
July 30, 2015 @ 11:29

the sequence is logical coz you will just assume the next number

Kaweesa says
August 3, 2015 @ 09:21

Answers without an explanation aren’t of much use.

mansur says
August 4, 2015 @ 02:44

139 says
August 6, 2015 @ 04:39

Both 17 and 3 they have definite numbers which the answer is zero

Bobson Natamba says
August 12, 2015 @ 16:11

Thenx for trying all of u bt the answer is 1

pro'c says
August 15, 2015 @ 05:10

the answer is just away from sequences its about some thing real like how many days are in the week?

Gimei Benedict says
August 17, 2015 @ 18:17

The number of the Sequence is 17. this means 17 is a number of the word SEQUENCE. therefore S=19 E=5 Q=17 U=21 E=5 N=14 C=3 E=5 according to the position of the alphabet. therefore the sequence is 19,5,17,21,5,14,3,5 If the second number is 3, which means 3 is the second number backwards, the first number is 19.

1 2