Example: In a certain school, the pupils have to study France, German, or English. Each pupil must study at least one of the three. Among a group of 100 pupils, 42 are studying France, 45 are studying German, 65 are studying English, 15 are studying France and German, 20 are studying France and English, 25 are studying German and English. Find the number of pupils who are studying all the three subjects. And Find the number of pupil who are studying English only.