Twelve cells each having the same e.m.f are connected in series and are kept to a closed box. Some of the cell are connected in reverse order .The battery is connected in series with an ammeter an external resistance `R` and two cells of the same type as an in the battery .The current when they and such each other is `3` ampere and current is `2` ampare when the two oppose each other. How many cells are connected in servese order ?