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

this is because when 40 is broken down into two numbers (one divisible to get out of the square root, and one not) you get 25 + 15 under the square root. we know that the square root of 25 is 5, so we can put 5 outside the root and leave 15 inside.
-------------- try the next problem, and if you don't understand, please reply back.
answer is for a) 
Answer:
The probability that you will pull a banana on the first try and an apple on the second try = .024
Step-by-step explanation:
Given -
A refrigerator contains 50 pieces of fruit: 6 apples, 5 oranges, 10 bananas, 3 pears, 7 peaches, 11 plums, and 8 mangos .
Total no of fruit = 50
The probability that you will pull a banana on the first try and an apple on the second try = 
= 
= .024
Answer:
it goes up to thirty
it has millimeters displayed in between