Hey there! I'm happy to help!
Let's look at each spot in the line from left to right and see how many choices they are.
In spot 1, we have 4 choices. Let's say that we choose the student who doesn't want to stand next to to the other student. This means that in the next slot, there are only 2 choices because he cannot stand by the other student. Now when we move to spot number three, there are only two choices left, and then there is 1.
4×2×2×1=16
So, there are 16 different ways the students can stand in a straight line if two of the students refuse to stand next to each other.
Have a wonderful day! :D
