Four cups of a salad blend containing 40% spinach is mixed with an unknown amount of a salad blend containing 55% spinach. The resulting salad contains 50% spinach.

How many cups of salad are in the resulting mixture?

8

9

12

13

The resulting mixture contains 12 cups of salad. The calculations for this problem involve material balances that can be derived from the information given above. The two equations for the two unknowns (amount of 55% spinach salad and the resulting salad) are:

Overall material balance: 4 + x = y

Spinach balance: 4 (0.4) + 0.55x = 0.5y

substituting both equations gives 8 cups of the 55% spinach salad, and 12 cups of the resulting salad.