Final answer:
Omilia needs 9.5 more cups of water.
Step-by-step explanation:
To find out how many more cups of water Omilia needs, we first need to determine the total amount of flour-water mixture she is making. Since the ratio of flour to water is 1:3, for every 1 cup of flour, she needs 3 cups of water. She starts with 4 cups of flour, so she should mix 4 x 3 = 12 cups of water in total.
Next, we subtract the amount of water she has already mixed (2.5 cups) from the total amount she needs (12 cups):
12 cups - 2.5 cups = 9.5 cups
Therefore, Omilia needs 9.5 more cups of water.