For histograms frequency represents area. For the 10-15 class the frequency is 15 and the area is 10 so for every 1.5 students there is i area unit. The base of this rectangle is 6 (15.5-9.5) so on the x axis there so they are using 3 x units per div
For the 16-18 class the frequency is 9 so the area will be 6The base ishalf of 6 cos the interval (18.5-15.5) is half of 6 ie bas is one so height is 6