Compressive strength interval	Freq	Rel freq	Rel freq density	Cumulative rel freq	Cumulative rel (%)
40-50       	     		20   	0.10   		0.010    		0.10   	       		10
50-55   			30   	0.15   		0.030  			0.25   			25
55-60   			40   	0.20   		0.040   		0.45   			45
60-65   			50   	0.25   		0.050   		0.70   			70
65-70   			30   	0.15   		0.030   		0.85   			85
70-80   			30   	0.15   		0.015   		1.00   			100