If a blacksmith uses seven nails in putting a shoe on a horse's foot, and receives cent for the first nail, cents for the second nail, and so on, what does he receive for putting on the shoe?
If a boy receives cents for his first day's work, cents for his second day, cents for the third day, and so on for days, what will his wages amount to?
If the population of a city is , and increases % a year for four years, what will be its population at the end of the four years? (Here .)