Hi,
I was attempting some psychometric tests and came across these 2 sequence questions that I couldn't figure out.

Sequence 1:
2 ? 4 6 4
What is the missing number?
The answer is supposed to be 2 according to the book.

Sequence 2:
9 16 178 ? 13259
What is the missing number?
The answer is supposed to be 1025.

I can't understand the patterns in the 2 sequences so if anyone could help me out, that would be great :D

Thanks.

Quote:

---

Originally Posted by val1930

Hi,
I was attempting some psychometric tests and came across these 2 sequence questions that I couldn't figure out.

---

Quote:

---

Sequence 1:
2 ? 4 6 4
What is the missing number?
The answer is supposed to be 2 according to the book.

---

This is one solution that works, though it is rather abstract. These are the first 5 Euler Phi numbers of the first 5 composite numbers.

The Euler Phi function or Totient function, , is the number of coprimes less than or equal to a given number. The first 5 composite numbers are 4,6,8,9,10
4 has 2 coprimes, 6 has 2 coprimes, 8 has 4 coprimes, 9 has 6 coprimes, and 10 has 4 coprimes.
Thus, the sequence 2,2,4,6,4
You can also find them by using



Where the p's are the prime factors of n.