SOLUTION: PLEASEEE HELPPP Einshtain once said: '1/5 of my students study maths, 1/7 study chemistry, 20 students doesn't study at all' How many students does he have?

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Question 436355: PLEASEEE HELPPP
Einshtain once said: '1/5 of my students study maths, 1/7 study chemistry, 20 students doesn't study at all'
How many students does he have?
Found 2 solutions by rfer, rwm:
Answer by rfer(16322) About Me  (Show Source):
You can put this solution on YOUR website!
7/35+5/35+20=
12/35+20=
20/0.657=30 students

Answer by rwm(914) About Me  (Show Source):
You can put this solution on YOUR website!
Your friend Einshtain needs to learn English.
20 students don't study at all.

Where did the other tutor get 35?
1/5*30=6
1/7*30=4 2/7
I have never seen 2/7 of a student walking around campus.