Hello,
We want to find how many ounces of total mixture Barbara has. If she has 54 ounces of granola and 36 ounces of raisins, she has 54 + 36 = 90 ounces of total mixture.
If she has 90 ounces of total mixture, and wants to make 6 ounce servings, we divide the total amount of mixture by the size of the servings.
Doing this, we get
90 / 6 = 15 servings.
She can make 15 servings.
Hope this helps, enjoy your lunch!