First, since we do not know what the costs are for a hot dog, fries, and a small drink, we will just use a,b,c represented the cost for each of the item.
a = cost of a hot dog
b= cost of the fries
c = cost of a small drink
From all the information that you get from the questions, you can get the following relations for a,b,c.
then you can solve for a, b, and c
add the first equation to second equation, we will get
from the second equation, you know that b+c=1.65
therefore 2a = 1.7
You will get a = 0.85