[(60+0)/2]*3+[(60.5+70.5)/2]*6+[(70.5+80.5)/2]*9+[(80.5+90.5)/2*]17+[(90.5+100.5)/2*]15=90+393+679.5+1453.5+1432.5=4048.5
4048.5/50=80.97
[(60+0)/2]*3+[(60.5+70.5)/2]*6+[(70.5+80.5)/2]*9+[(80.5+90.5)/2*]17+[(90.5+100.5)/2*]15=90+393+679.5+1453.5+1432.5=4048.5
4048.5/50=80.97