what formula should i use also

(3) PROBLEM SOLVING WITH PERCENTS: You are planning a wedding re- ception for 75 guests that will take place in exactly 10 months. There are two different sites that you are contemplating using, with the stipulations on payments as described below. Determine the TOTAL cost of renting each location. (a) Rental of space = $68 per guest; Downpayment = 25% of total cost; Payment due one month before the reception includes a 7.4% interest payment on the balance. 68 Guest 20%+(18- guest - Clown Payment) (b) Rental of space $70 per guest; Downpayment = 50% of total cost; Payment due one month before the reception includes a 2.0% interest payment on the balance