If I won 72 billion on slots and won 120 times my spin. The average bet is 50 coins per spin. How much was I betting per spin? I thought the answer was 600 million but it's not. Help?

Maria donates a fixed amount, a, to a charity each month. if she donates $300 in 12 months, what is the equation for a? a. a + 300 = 12 b. a × 300 = 12 c. a × 12 = 300 d. a + 12 = 300 e. a + 32 = 100