Add up all the whole numbers from 1 to 100 except those that divide exactly by 5. What total do you get?

The whole run from 1 to 100 adds to $100\times 101÷2=5050$. The multiples of 5 in that run are 5, 10, 15, … , 100, which is 5 × (1 + 2 + … + 20) = $5\times 210=1050$. Taking those out leaves 5050 − 1050 = 4000. The traps: 5050 forgets the subtraction altogether; 4050 comes from guessing the multiples of 5 add to a round 1000; and 3950 subtracts 1100, that is, one multiple of 5 too many.