Answer:
The first thing you should do for this case is to graph the data you have in the statement to see the relationship between grapes and blueberries.
The function represented by the table is
y = 0.25x
Then, as Mandie is making 15 glasses of fruit salad, we have:
y + x = 15
We have two equations and two unknowns that we must solve.
y = 0.25x
y + x = 15
Resolving
For x:
0.25x + x = 15
1.25x = 15
x = 15 / 1.25 = 12
For y:
y = 0.25x = 0.25 (12) = 3
Answer
The ratio of grapes to blueberries she should use is 12: 3