It's hard to understand with equations. Let's calculate directly:

The difference between 5 and 6 is 1.

How to understand it? Buy five or six more, and the difference will double.

Now the smallest difference is the one with one more point and the one with two less points.

That is 1+2=3.

So it's three times.

5*3+ 1= 16, which can also be expressed as 6*3-2= 16.

So it is 16.

Overall process:

1+2=3

5*3+ 1= 16 (or 6*3-2= 16)

A: I bought at least a gift of 16.

Note: "*" indicates the multiplication symbol.