How many ways can you make a sandwich by choosing 4 out of 10 ingredients?

Step 1
While there is a formula to use, it is quite easy to think through the question as follows:
When we choose the first ingredient, there are 10 choices.
When we now choose the second ingredient there are 9 choices.
For the third, there are 8.
For the fourth there are 7.
Step 2
Each ingredient can be combined with the others, giving: ${\displaystyle 10\times 9\times 8\times 7=5,040}$.