10 m = 10 * 10 = 100 dm
100 / 3 = 33.3333
33 pieces of ribbon (each 3 dm long) can be cut from a 10m piece of ribbon
in general:
How many x dm pieces of ribbon can be cut from a y m piece of ribbon?
If 10y > or = x, then 10y/x pieces, dropping any decimal remainder
If 10y < x, then 0 pieces can be made, because the cut piece is longer than the starting piece.