quince
/ˈkinθe/ · B1 · num
fifteen
From Latin quindecim: quinque (five) + decem (ten) = fifteen
The quin- in quince is the same five as in quintet and quintuplets, and -ce is what's left of decem, ten. Five plus ten is fifteen. Once (11) to quince (15) all end in -ce for the same reason.