VTMO 2026 - Grade 4 - Semifinal - SET 12 - Self-study Question 4

An, Bình and Châu have 90 marbles altogether. An gives Bình 12 marbles, Bình gives Châu 13 marbles, and Châu gives An 5 marbles. They then have equal numbers of marbles. How many did An have at first?