so he paid x then he added 1/5 of the price to the motorcycle and sold it for 13,800 or 6/5 of the origonal price
if 6/5 of origonal price=13,800
you divide 13,800 by 6 to get the amout he marked it up by ([6/5]/6 or 6/5*6 or 1/5) so 2300 is the amount that he marked the price up by so 13,800-2300=origonal price=$11,500