The question you opened
The digit that grows upside down
I am a single digit. Turned upside down I become 3 bigger.
What digit am I?
6
Six turned upside down reads as nine, and nine is three more than six. No other digit changes into a bigger one.
I am a single digit, and turned upside down I become three bigger. The answer is 6, because an upside down 6 reads as a 9, and 9 is three more than 6. This one is solved by looking rather than calculating. Write all ten digits and turn the page around. Most of them turn into nothing readable. Zero stays zero. One stays one. Eight stays eight. Two, three, four, five and seven become shapes that are not digits at all.
Only six and nine swap places with each other. So the pair is fixed, and the only question left is direction. Six becomes nine, which is bigger by three. Nine becomes six, which is smaller by three. The clue says bigger, so the answer is 6. The method here is worth naming: when the list of possible answers is short, check all of them. There are only ten digits. Testing every one takes about ten seconds and leaves no room for doubt.
Puzzles feel hard when you try to be clever, and many of them fall over immediately to a complete list. The six and nine pair causes real trouble outside puzzles. You will sometimes see a small line printed under a 6 or a 9 on a shirt or a playing card, put there so nobody argues about which way up it goes. Digital clocks and calculators avoid the problem by drawing the digits with straight segments, which is why an upside down calculator spells words instead of numbers.
You can build your own version of this riddle. Ask for a two digit number that becomes 45 bigger when turned upside down. Sixteen becomes 91, and 91 minus 16 is 75, so that is not it. Try 69, which becomes 69, and the difference is zero. Try 68, which turns into 89, and 89 minus 68 is 21. Hunting for the right pair is more fun than being told, so leave the answer to whoever you ask.
Technique: Try every digit, there are only ten