The question you opened
The number that hides two clues
I am a two digit number. My tens digit is twice my ones digit, and my digits add to 9.
What number am I?
63
One digit is twice the other, so the 9 splits into three equal shares of 3. The digits are 6 and 3, giving 63.
I am a two digit number. My tens digit is twice my ones digit, and my digits add up to 9. The answer is 63. Here is a way to see it without guessing. The ones digit is one share. The tens digit is twice that, so it is two shares. Together they are three shares, and together they add to 9. So one share is 3. The ones digit is 3 and the tens digit is 6, which makes the number 63.
Guessing works too, and it is a perfectly good method when the list is short. Numbers whose digits add to 9 are 18, 27, 36, 45, 54, 63, 72, 81 and 90. Run down the list and check the second clue on each: is the first digit twice the second. For 63 it is, because 6 is twice 3. For 36 it is not, because 3 is half of 6. That near miss is the interesting part.
Digit puzzles nearly always have a mirror answer that satisfies one clue and fails the other, and it sits right next to the real answer. Checking every clue before deciding is the whole skill. The equal shares idea is worth keeping for other questions. If two things are described as one being twice, or three times, another, and you know their total, then split the total into that many equal pieces.
Sharing 24 sweets so one person gets three times the other means four shares of 6, so 18 and 6. Same move every time. Once you have an answer, put it back into both clues out loud. Six is twice three. Six plus three is nine. Both true, so 63 is the number. Try inventing one of your own. Pick two digits, write down two facts about them, and see whether a friend can find the pair.
Technique: Split the total into equal shares