On a piece of paper certain odd number of horizontal lines at equal distances are drawn. An equal number of vertical lines at equal distances are also drawn, forming rectangles. The rectangles are coloured black or white, set alternately. Three coins are put at random in these rectangles. Let p be the probability of having two coins in rectangles of the same colour and the third in a rectangle of the other colour, and p' be the probability of having all the coins in the rectangles of the same colour. If `p : p' =16 : 5` then find the number of horizontal lines drawn on the paper.