Scale = proportion,
15 cm on the map correspond to 60 m, so the proportion is 60*100= 6000 cm/15cm = 400, notice the same units (!)
Then for the other length: 12 cm * 400 = 4800 cm or 48 m.
Another way, if 15 cm are 60 m, then 12 cm are:
12/15*60 = 48 m
As you prefer!