We want to find how many times 1 1/2 cups fits in to 48 cups. In other words, we need to divide 48 by 1 1/2.
To make easier, let's convert 1 1/2 in to a proper fraction:

Now we want to divide 48 by 3/2. Remember that divide by a fraction is the same as multiply by the inverse:

Now solve:

Thus, the amount of serving with 48 cups is 32.