Using six 9s and any math operations (+, -, ×, ÷, parentheses), make 100.
I have as many brothers as sisters. But each of my sisters has twice as many brothers as sisters. How many boys and girls are in my family?
How many seconds are in a year? (No calculator!)
Three friends share a $30 hotel room (each pays $10). The clerk realizes the room costs only $25 and gives $5 back. The bellhop keeps $2 for himself and returns $1 to each person. So each person paid $9 (total: $27) and the bellhop has $2. Where's the missing dollar?