This is a very simple question (If we understood well).

Here is the answer:

We want to calculate how much is 30 percent of 400.

To achieve this, we just have to multiply 400 by 30 and then divide this partial result by 100.

So, the number of senior taking Math is

^{(400 x 30)} / _{100} = 4 x 30 = 120 (answer)