I need a clearer understanding of how to solve: Prince Thaddeus slew 100 dragons with his mighty sword, while Prince Arthur, using a spear, slew three quarters as many dragons as Prince Thaddeus.  Using a rusty iron axe, Prince Walter slew twice as many dragons as Prince Arthur.  But Prince Bruce, having forgot his sword at home, slew one-fifth as many dragons as Prince Walter using a nail file.  How many Dragons has Prince Bruce slain?