Nitrogen and hydrogen join in the ratio 14 : 3 by mass to make ammonia.

A compound always has its elements in the same ratio by mass: water $1 : 8$, carbon dioxide $3 : 8$, ammonia $14 : 3$, magnesium oxide $3 : 2$. To share a mass in a ratio, add the parts first. Any extra of one element is left over.