Work out the value of 100 − 99 + 98 − 97 + 96 − 95 + … + 4 − 3 + 2 − 1.

Bracket the numbers in pairs from the left: $(100-99)+(98-97)+(96-95)$ + … + (2 − 1). Each bracket comes to 1, and since the 100 numbers make 50 brackets the total is $50\times 1=50$. The traps: 0 is the guess that the pluses and the minuses cancel out completely, which would need the two sides to be equal — but every plus number is one bigger than the minus number beside it; 100 counts a bracket for every number rather than for every two numbers; 1 is the value of a single bracket, with the other 49 forgotten.