Steps
So remember that ratios are part to part, not part to whole like fractions are. In this case:
5 + 3 + 4 = 12
There are 12 parts total. This means that Pru has 5/12 of the spices, Piper has 3/12 (or 1/4) of the spices and Phoebe has 4/12 (or 1/3) of the spices. Now, we also know that Pru has 24 more spices than piper. With this info, we can set up our equation as (x = total amount of spices):

From here we can solve for the total amount of spices. Firstly, subtract both sides by 3/12x:

Next, multiply both sides by 12/2 (or 6):

There are a total of 144 spices. Now, to get Phoebe's amount of spices multiply 1/3 by 144:

Answer:
In short, Phoebe has 48 spices.