Answer:
if she has to buy from a mixture of both the cheeses then equation will become;
N x C + M x S = 30
Explanation:
Let C represents the value of cheddar cheese per pound
Let S represents the value of swiss cheese per pound.
Equation when she only buys cheddar cheese will be;
N x C = 30,
Here N is the number of pounds of cheddar cheese
Equation when she only buys swiss cheese will be;
M x S = 30,
Here M is the number of pounds of swiss cheese
Now if she has to buy from a mixture of both the cheeses then equation will become;
N x C + M x S = 30