ANSWER: 
The cost of two walnuts varies proportionality with weight.The equation is c = 2.5w
SOLUTION: 
Given, the cost of two walnuts varies proportionality with weight a two pound bag costs $5, a five pound bag costs $12.50. 
Let the proportionality constant be x. 
According to the given information, 
c = xw --- (1) 
Where, c is cost in dollars. 
x is proportionality constant 
w weight of walnuts in pounds 
Given that, a two pound bag costs $5 
Here, c = 5 and w = 2 
So, substitute above values in (1) 
5 = x(2) 
x = 

x = 2.5 
We have another case also, so let us check it also. 
A five pound bag costs $12.50 
Here, c = 12.50 and w = 5 
So, substitute above values in (1) 
12.5 = x(5) 
x = 
 = 2.5
 = 2.5 
So the value of proportionality constant is 2.5 
Hence, the equation is c = 2.5w