Add Reversed Numbers
Algebraic shortcut: ab + ba = 11(a+b)
47 + 74 = 121
Split both: 47 is 40 + 7, and 74 is 70 + 4 — the same digits, swapped.
Every digit lands once in the tens and once in the ones, so it is worth 11.
So add the digits and multiply by 11: 4 + 7 = 11, and 11 × 11 = 121.
Your turn
Three to try in your head — tap what you get.
78 + 87
34 + 43
45 + 54
All 36 shortcuts: the Magic Tricks library.