Explanation:
Draw a three-ring Venn diagram.
7 students study all three subjects, where the three circles overlap.
12 students study biology and chemistry, which includes the 7 who also study physics. So there are 5 who study biology and chemistry but not physics.
Similarly, 15 students study biology and physics, which includes the 7 who also study chemistry. So there are 8 who study biology and physics but not chemistry.
And, 20 students study chemistry and physics, which includes the 7 who also study biology. So there are 13 who study chemistry and physics but not biology.
35 students study biology, which includes the 5 who study chemistry, the 8 who study physics, and the 7 who study both chemistry and physics. Therefore, there are 15 students who study only biology.
Similarly, 32 students study physics, which includes the 8 who study biology, the 13 who study chemistry, and the 7 who study both biology and chemistry. Therefore, there are 4 students who study only physics.
Since there's a total of 60 students, there must be 8 students who study only chemistry.