If there is 4 student and all the student ate 1/4 of the cookies they bake and they had 81 leftover. How many cookies did they bake?

\(\text{Let student }k \text{ have baked }n_k \text{ cookies}\\ \text{the }{n_k}'s \text{ might not all be equal}\\ N = \sum \limits_{k=1}^4 ~n_k= \dfrac 1 4 N+81\\ 81 = \dfrac 3 4 N\\ N=108\)