24 题 1/2 of students study biology, 1/3 students study chemical. after one year, 1/3 of the students who study biology graduated, 1/4 of students who study chemicals graduated. there are no new students come in. ask the fraction of the students who study biology or chemical. answer should be 7/9 or something.
这题为什么不考虑bio 和 che 都学的情况呢?