. 3 6 1 1 - 1 . ▁This ▁represents ▁an ▁imp lied ▁probability ▁of ▁ 2 9 . 7 5 2 % ▁( 1 / 3 . 3 6 1 1 ) ▁and ▁multip lying ▁by ▁ 4 ▁( for ▁each ▁of ▁the ▁four ▁equally ▁likely ▁combinations ▁of ▁out comes ) ▁gives ▁a ▁total ▁book ▁of ▁ 1 1 9 . 0 1 %. ▁Thus ▁the ▁over round ▁has ▁slightly ▁more ▁than ▁double d ▁by ▁combining ▁two ▁single ▁bet s ▁into ▁a ▁double . ▁ ▁In ▁general , ▁the ▁combined ▁over round ▁on ▁a ▁double ▁( OD ), ▁expressed ▁as ▁a ▁percentage , ▁is ▁calculated ▁from ▁the ▁individual ▁books ▁B 1 ▁and ▁B 2 , ▁expressed ▁as ▁dec im als , ▁by ▁O D ▁= ▁B 1 ▁× ▁B 2 ▁× ▁ 1 0 0 ▁− ▁ 1 0 0 . ▁In ▁the ▁example ▁we ▁have ▁O D ▁= ▁ 1 . 0 9 0 9 ▁× ▁ 1 . 0 9 0 9 ▁× ▁ 1 0 0 ▁− ▁ 1 0 0 ▁= ▁ 1 9 . 0 1 %. ▁ ▁This ▁massive ▁increase ▁in ▁potential ▁profit ▁for ▁the ▁book maker ▁( 1 9 % ▁instead ▁of ▁ 9 % ▁on ▁an ▁event ; ▁in ▁this ▁case ▁the ▁double ) ▁is ▁the ▁main ▁reason ▁why ▁book m akers ▁pay ▁bon uses ▁for ▁the ▁successful ▁selection ▁of ▁w inners ▁in ▁multiple ▁bet s : ▁compare ▁offering ▁a ▁ 2 5 % ▁bonus ▁on ▁the ▁correct ▁choice ▁of ▁four ▁w inners ▁from