Let's note the information.
This is what we know:
Every buttons came in packages of 30 per student.
Every bookmark came in packages of 50 per person.
Know, we need to do some multiplying.
30 x 400 & 50 x 400
Now solve.
30 X 400 = 12000.
So we now know that they have an estimated 12000 buttons total.
Now, we find x for bookmarks.
50 x 400 = 20000
That means total that estimated that 20000 bookmarks will be given.