I think the answer is 70a^4b^5c^4.
The answer is 1.52 to your question.
(2/5) of (2/3) of students = 4/15 of students study French. Then 1-(4/15) = 11/15 of students do not study French. The ratio you want is
(4/15):(11/15) = 4:11
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In a math expression "of" means "times".