I believe it would be D. a = 100 - 16d.
Because you have 100 dollars overall and you spend 16 dollars per day.
If you plug 4 (for the number of days that 16 dollars was spent) into the equation, you will find out if you have enough money left. Let's see!
a = 100 - 16d
a = 100 - 16(4)
a = 100 - 64
a = 36
Yay! You have exactly 36 dollars left! You can buy the sweatshirt!
Hope this helps!
