Think of a number. Add 8 to it, multiply the answer by 5, then take away 20. The result is 100. What number was thought of?

Work backwards, undoing each step in reverse order: 100 + 20 = 120, then 120 ÷ 5 = 24, then 24 − 8 = 16. Checking forwards, 16 + 8 = 24, $24\times 5=120$, 120 − 20 = 100. The traps: 24 stops one step early and forgets to undo the 'add 8'; 8 undoes the last step by subtracting 20 again instead of adding it back; 32 undoes the steps in the original order rather than in reverse.