Mary sells lemonade, and she donates 10 cents from every cup she sells to charity. The cups of lemonade sell for 45 cents. One day she made $17.10. How much money did she give to charity that day?

First, find out how many times 45 will go into 17.10. to find out how many cups she sold

17.10/45 = 38

and since she gives 10c to charity every cup you will have to divide to find the answer which is $3.80 that she gave to charity! : D