This is a slightly challenging problem from Dan Griller.
“Every pupil at the Euler Academy studies French or Spanish. At the start of the year, one third of the French students also studied Spanish, and 2 fifths of the Spanish students also studied French. After one term, six of the double-linguists dropped French, so that now only a quarter of the French students study Spanish. How many pupils are at the Euler Academy?”
Just to be clear, “French students” means Euler Academy pupils studying French, and similarly for “Spanish students.”
See the Language Students Puzzle