The pizza parlor is running a special on 3-toppings pizzas. The topping choices include pepperoni, sausage, bacon mushrooms, onions, green peppers, and black olives. The next customer who orders 3-topping pizza tells the chef to randomly choose 3 different toppings for their pizza. What is the probability that customer will get a pizza topped with pepperoni, mushrooms, green peppers? The teacher told us the solution is 1/35 but he wants an explanation of why is that solution correct.