There are three pillars $X$ , $Y$ and $Z$ of different heights. Three spiders $A$ , $B$ and $C$ start to climb on these pillars simultaneously. In one chance, $A$ climbs on $X$ by 6 cm but slips down 1 cm. $B$ climbs on $Y$ by 7 cm but slips down 3 cm. $C$ climbs on $Z$ by 6.5 cm but slips down 2 cm. If each of them requires 40 chances to reach the top of the pillars, what is the height of the shortest pillar?