In a certain classroom, 75% of the students can speak English, 55% of the students can speak French, and 10% speak neither language.

What percentage of the students speak both languages?

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40%

You can think of the whole group of students as comprising the 75% of students that speak English, the 55% of students that speak French, minus the x% of students that speak both languages (because this group has already been counted twice, we subtract to avoid double-counting), plus the 10% the speak neither:

100 = 75 + 55 – x + 10

Solving this equation gives us x = 40.