There are 78 prisoners in a square cell block of twelve cells. There is 1 prisoner in one of the cells, 2 in another cell, 3 in another, 4 in another and so on up to 12 prisoners in one of the cells. The clever prison warder made it easy to check if the prisoners were all there by arranging them so there were 25 along each wall of the prison block. How did he do it?

(There's more than one solution - send yours in - it might be different to everyone else!)