21 and 42 1

Logic Level 4

A set 'A' is defined to contain any positive integer which has the property of being a string of "21's", "42's", and/or "421's". If all the integers in set A are ordered from smallest to largest, what is the 21st term of this sequence?

For clarification, the sequence starts out: 21, 42, 421, 2121, etc.


The answer is 2121421.

Pickle Lamborghini by pickle lamborghini , 29, USA

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

The sequence: (1) 21, (2) 42, (3) 421, (4) 2121, (5) 2142, (6) 4221, (7) 4242, (8) 21421, (9) 42121, (10) 42142, (11) 42421, (12) 212121, (13) 212142, (14) 214221, (15) 214242, (16) 421421, (17) 422121, (18) 422142, (19) 424221, (20) 424242, (21) 2121421 \boxed{2121421}

0 Helpful 0 Interesting 0 Brilliant 0 Confused

For Easier Reading

21

42

421

21 21

21 42

42 21

42 42

21 421

421 21

421 42

424 21

21 21 21

21 21 42

21 42 21

21 42 42

421 421

42 21 21

42 21 42

42 42 21

42 42 42

21 0 21 0 421 \boxed{21\phantom{0}21\phantom{0}421}

John Wyatt - 5 years, 5 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...