▁the ▁usual ▁decimal ▁system . ▁One ▁modern ▁method ▁of ▁finding ▁the ▁extra ▁needed ▁symbols ▁is ▁to ▁write ▁ten ▁as ▁the ▁letter ▁ ▁( the ▁ 2 0 ▁means ▁base ▁), ▁to ▁write ▁nin ete en ▁as ▁, ▁and ▁the ▁numbers ▁between ▁with ▁the ▁corresponding ▁letters ▁of ▁the ▁alphabet . ▁This ▁is ▁similar ▁to ▁the ▁common ▁computer - s cience ▁practice ▁of ▁writing ▁hex ade c imal ▁numer als ▁over ▁ 9 ▁with ▁the ▁letters ▁" A – F ". ▁Another ▁less ▁common ▁method ▁ski ps ▁over ▁the ▁letter ▁" I ", ▁in ▁order ▁to ▁avoid ▁confusion ▁between ▁I 2 0 ▁as ▁eigh teen ▁and ▁one , ▁so ▁that ▁the ▁number ▁eigh teen ▁is ▁written ▁as ▁J 2 0 , ▁and ▁nin ete en ▁is ▁written ▁as ▁K 2 0 . ▁The ▁number ▁twenty ▁is ▁written ▁as ▁. ▁ ▁Con ver ting ▁table ▁▁ ▁According ▁to ▁this ▁notation : ▁ ▁means ▁forty ▁in ▁decimal ▁= ▁▁ ▁means ▁two ▁hundred ▁and ▁si xty ▁in ▁decimal ▁= ▁▁ ▁means ▁four ▁hundred ▁in ▁decimal ▁= ▁. ▁ ▁In ▁the ▁rest ▁of ▁this ▁article ▁below , ▁numbers ▁are ▁expressed ▁in ▁decimal ▁notation , ▁unless ▁specified ▁otherwise . ▁For ▁example , ▁ 1 0 ▁means ▁ten , ▁ 2 0 ▁means ▁twenty . ▁Num bers ▁in ▁v iges imal ▁notation ▁use ▁the ▁convention ▁that ▁I ▁means ▁eigh teen ▁and ▁J ▁means ▁nin ete en . ▁ ▁Fra ctions ▁ ▁As ▁ 2 0 ▁is ▁divis ible ▁by ▁two ▁and ▁five ▁and ▁is ▁adjacent ▁to ▁ 2 1 , ▁the