# Difficult number sequence

33, 66, ?, 1212, ?, 1218

I just can't figure out how to complete this sequence. The last number throws off everything I try. Anyone have some ideas? I can perhaps see the missing numbers ending in 9 and 15, but that still might be the wrong pattern.

Taken from: https://news.generiq.net/Trilogica/algebrica.html

• They make a few handful of these questions (32, to be exact), label it "IQ test" and is asking 35-40€ to grade the results? Good lord... – votbear Mar 27 '18 at 6:43
• @votbear it's pretty simple, if you are dumb enough to pay, it means you have failed the test – Kepotx Mar 27 '18 at 6:56
• Related joke – A J Mar 27 '18 at 7:08
• If you got this from somewhere the most obvious answer is that the last number is a misprint. – Willtech Mar 27 '18 at 9:22

I think this is it.

Goes up in multiples of $3$ and number of each colour increases each time. $$\color{red}{33},\color{blue}{66},\color{blue}{69},\color{green}{1212},\color{green}{1215},\color{green}{1218},\color{purple}{1521},\color{purple}{1524},\color{purple}{1527},\color{purple}{1530}$$.

or

First digit(s) doubles each time. $$\color{red}{33},\color{blue}{66},\color{blue}{69},\color{green}{1212},\color{green}{1215},\color{green}{1218},\color{purple}{2424},\color{purple}{2427},\color{purple}{2430},\color{purple}{2433}$$

• Would have upvoted if I didn't reach my daily limit :\ :D – Mr Pie Mar 27 '18 at 12:21
• @TheSimpliFire If you are correct isn't there a gap between the blue and the green colour? – rhsquared Mar 27 '18 at 12:41
• maybe purple should start at 2424? – Florian Bourse Mar 27 '18 at 12:44
• Looks plausible. – user632 Mar 27 '18 at 15:47
• I think in the second sequence, there should be 1 more green term - 1221 – Shivansh Sharma Mar 18 '20 at 9:20

I'm new here. Another solution could be

129 and 615. Basically, we sum up the digits of previous number and append the consecutive multiple of 3 as follows:

3+3=6 (6),
6+6=12 (9),
1+2+9=12 (12),
1+2+1+2=6 (15),
6+1+5=12 (18)

That'd make the next one

1221

• Welcome to Puzzling.SE! It's customary here to hide answers inside spoiler tags (type >! at the start of each line), to avoid spoiling the solution for anyone who wants to have a go at the puzzle themselves. That's a pretty clever solution, btw! Feel free to take the tour and visit the help center to learn more about the site, and I hope you stick around! – F1Krazy May 10 '18 at 7:47

The complete sequence is:

33 66 39 1212 1515 1218

This because:

33=first number of sequence