The question you opened
A crate of eleven melons
A crate holds 11 melons and each melon sells for 23 dollars.
What is the crate worth, in dollars?
$253
To multiply by 11, pull the digits apart and drop their sum in between. Two and three make five, so 23 becomes 253.
Multiplying by 11 has a shortcut that looks like a party trick and is really just addition. Take the two digits of 23, write the first and last with a gap between them, and put their sum in the gap. Two, then 2 plus 3, then 3, gives 253. When the middle sum passes 9, the extra carries left. For 67, the digits sum to 13, so write 6, then 3, and carry the 1 into the 6, giving 737.
For 85, the sum is 13, so 8 becomes 9 and the answer is 935. The carry is the only part that needs care. The reason it works is that 11 is 10 plus 1. Multiplying 23 by 11 is 230 plus 23, and adding a number to itself shifted one place is exactly what puts the digit sum in the middle.
Seeing it that way also gives the safer general method: to multiply by 11, add a zero and add the original. Two hundred and thirty plus 23 is 253, and that version never fails on three digit numbers where the digit trick gets fiddly. The same shape covers other near round multipliers. To multiply by 9, add a zero and subtract the original: 23 times 9 is 230 minus 23, which is 207.
To multiply by 12, add a zero and add double: 230 plus 46 is 276. To multiply by 99, add two zeros and subtract: 2,300 minus 23 is 2,277. Once these are automatic, a surprising share of everyday multiplication turns into one shift and one addition. Prices, weights, and quantities cluster near round numbers precisely because people set them, and that clustering is what makes the shortcuts pay.
Technique: Times 11: split, then add the middle