A salad bar offers 10 choices of toppings for lettuce. In how many ways can you choose three or four toppings?

To solve this problem, we will use the concept of combinations in mathematics.

1. First, we need to find out how many ways we can choose 3 toppings out of 10. This is a combination problem which can be solved using the formula for combinations: C(n, k) = n! / [k!(n-k)!], where n is the total number of options, k is the number of options to choose, and "!" denotes factorial.

   So, C(10, 3) = 10! / [3!(10-3)!] = 120 ways.

2. Next, we need to find out how many ways we can choose 4 toppings out of 10. Using the same formula:

   C(10, 4) = 10! / [4!(10-4)!] = 210 ways.

3. Since we can choose either 3 or 4 toppings, we add the two results together to get the total number of ways to choose toppings.

   So, 120 (from step 1) + 210 (from step 2) = 330 ways.

Therefore, there are 330 ways to choose either three or four toppings from a selection of 10.