Describe a situation that could be modeled with the ratio 4: 1

Each student in Mrs. Azulai's 4th period math class takes his collection of
$1 bills to his nearest neighborhood branch bank, and exchanges the bills
for 25¢ pieces.  During their excited discussion of their experiences in the
next day's class, the students are astonished to learn that no matter how few
or how many $1 bills any of them had, the ratio of the number of $1 bills to the
number of 25¢ pieces was precisely the same for each and every student.
Calculate that ratio .