A pizza parlor has six different toppings. How many different one- and two-topping pizzas can you order?
This problem is about counting the number of ways to choose one or two items from a set of six items. One way to approach this is to use the combination formula, which tells us how many ways to choose r items from a set of n items, regardless of the order of the selection. The combination formula is nCr = n!/(r!(n-r)!), where n! is the factorial of n, which means the product of all positive integers from 1 to n. To choose one topping from six toppings, we have 6C1 = 6!/(1!(6-1)!) = 6!/5! = 6 ways. To choose two toppings from six toppings, we have 6C2 = 6!/(2!(6-2)!) = 6!/(2!4!) = (6*5*4!)/(2*4!) = 15 ways. So the total number of different one- and two-topping pizzas is 6 + 15 = 21. 