Take any whole number and apply the rules and play.

Select any whole number.

If it is an even number, divide by 2; if it is odd number multiply by 3 and add 1.

Repeat the process mentioned in step 2 until you get the loop value 4, 2, 1 in repetition.

Whole number is 15. 15 is an odd no; so (15 × 3) + 1 = 46 46 is an even no; so 46 / 2 = 23 23 is an odd no; so (23 × 3) + 1 = 70 70 is an even no; so 70 / 2 = 35 35 is an odd no; so (35 × 3) + 1 = 106 106 is an even no; so 106 / 2 = 53 53 is an odd no; so (53 × 3) + 1 = 160 160 is an even no; so 160 / 2 = 80 80 is an even no; so 80 / 2 = 40 40 is an even no; so 40 / 2 = 20 20 is an even no; so 20 / 2 = 10 10 is an even no; so 10 / 2 = 5 5 is an odd no; so (5 × 3) + 1 = 16 16 is an even no; so 16 / 2 = 8 8 is an even no; so 8 / 2 = 4 4 is an even no; so 4 / 2 = 2 2 is an even no; so 2 / 2 = 1 1 is an odd no; so (1 × 3) + 1 = 4 4 is an even no; so 4 / 2 = 2 2 is an even no; so 2 / 2 = 1 So the loop 4..2..1 goes on and on.

Tip : The angel number 421 is the smallest prime formed by the two powers in logical order from right to left.