Math trick
Ask someone to name a two-digit number. Let’s say the person calls out 13. You then write down 13 and keep writing consecutive numbers until you have 10 numbers in total:
13, 14, 15, 16, 17, 18, 19, 20, 21, 22
What’s the sum of these numbers? All you need to do is find the 5th number from the left, which is 17. Then simply put a 5 to that number to get 175—and that’s the sum!
Let’s try another example. Suppose for 69. You write:
69, 70, 71, 72, 73, 74, 75, 76, 77, 78
The 5th number from the left is 73. Put a 5 to get 735—and that’s the sum!
This trick works for any positive whole number. You can even try it with three-digit numbers, like 420:
420, 421, 422, 423, 424, 425, 426, 427, 428, 429
The 5th number from the left is 424, and appending a 5 gives 4245, which is the sum.