The Freshman fifteen refers to students in their first year of college gaining 15 lbs (6.8 kg) for various reasons. It was joked about enough for even me to be aware of it when I was in college.

I have NO idea if it's remained commonly joked about thing, or what all countries it's joked about in (is it the freshmen 15 kg, or 33 lbs., in Europe?). Either way, here it is in a comic I have made in 2024.

Nanase originally said "fifty," but I thought "one-fifty" sounded better.

Fifty is quite a bit more than fifteen, but "freshmen fifty" somehow sounded like a downgrade from "freshmen fifteen"? I guess it is a shorter word, and shorter words weigh less. That difference had to be made up for by making Nanase hypothetically gain thrice as much weight.

It's just basic linguistic math, really.