KöMaL 2006 K88

There are four ninth-grade classes at a school. The four classes went on a four-day camp. Each class went hiking on one of the four days, while the other classes took part in presentations and other activities. On Monday, class $A$ went hiking and thus 81 students remained in the camp. On Tuesday, class $B$ were hiking and thus 79 students remained in the camp. On Wednesday, class $C$ were hiking and 75 students stayed in the camp. Finally, on Thursday, with class $D$ hiking, 80 students stayed in the camp. How many students are there in each class?