The bus fare in a city is $ 2.00. People who use the bus have the option of purchasing a monthly coupon book for $ 20.00. With the coupon book, the fare is reduced to $ 1.00 .

Determine the number of times in a month the bus must be used so that the total monthly cost without the coupon book is the same as the total monthly cost with the coupon book. The bus must be used ____ times.