I don't think you're stupid. ^^ Let's take the third term, 8. If n=3, then an should be 8. The first answer gives 15+2.5*3=22.5. The second answer gives 15.5+2.5=23. The third answer gives 15-2.5*3=7.5. The fourth answer gives 15.5-2.5*3=8. Choice 4 is correct.
Let's try to actually find the equation on our own. The equation for an arithmetic series is an=a0+dn where a0 is the term that comes before the first term. The first term given is 13 and it decreases by 2.5 each time so n=-2.5 and the term that would be before 13 is 15.5. Thus the equation is an=15.5-2.5n
