Question

What is the next number of the following sequence
54, 21, 3, 9, 81,......

Answer 1

2nd digit ² +first digit = next number
4²+5=21
1²+2=3
3²+0=9
9²+0=81
1²+8=9
So the next number is 9.

Answer 2

square the second digit and add it with the first digit, you will get the number.
In 54, the second digit is 4, and the first digit is 5.
Therefore, 4²+5
=16+5
=21
Similarly, 1²+2
=1+2
=3
Similarly, 3²+0
=9+0
=9
Similarly, 9²+0
=81+0
=81
Similarly, 1²+8
=1+8
=9
Thus, you will get the answer as 9.