Let n be the number of novels and m be the number of magazines that Sadie purchased. If Sadie purchased a total of $11 novels and magazines, then n+m=11. It is given that the combined price of 11 novels and magazines is $20. Since each novel sells for $4 and each magazine sells for $1,it follows that 4n+m=20. So the system of equations below must hold
4n+m=20 n+m=11
Subtracting side by side the second equation from the first equation yields 3n=9, so n=3. Therefore, Sadie purchased 3 novels. Choice A is incorrect. If 2 novels were purchased, then a total of $8 was spent on novels. That leaves $12 to be spent on magazines, which means that 12 magazines would have been purchased. However, Sadie purchased a total of 11 novels and magazines. Choices C and D are incorrect. If 4 novels were purchased, then a total of $16dollars was spent on novels. That leaves $4 to be spent on magazines, which means that 4 magazines would have been purchased. By the same logic, if Sadie purchased 5 novels, she would have no money at all ($0) to buy magazines. However, Sadie purchased a total of 11 novels and magazines.