Question: how many integers between 1 and 10^21?
Answer: 000.000 (=10^21) has 1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21 =231 which is exactly the correct answer. However, the OFFICIAL answer mentions this solution: 10^21 is a 22 digit number, this means that all integers betweeen 1 and 10^21 will have at most 21 digits.