Why did he take the average this way? He could just add all the numbers to each other than multiplies by their count. And when I did it I got a different answer. (10+15+13+9+32+10+25+0)/8= 6982.5. I would really appreciate an explanation.

the scale is 1:7,500 1CM on the map =7500CM in arears 1CM=75CM Step 2/4 (i) Average height rule: Take average height of two sides for each area h1=210+15=12.5cmh2=215+13=14cmh3=213+9=11cmh4=29+32=20.5cmh5=232+10=21cmh6=210+25=17.5cmh7=225+0=12.5cmthereforetotalareaincm2=h110+h210+h310+h410+h510+h610+h710area=12.570+1470+1170+20.570+2170+17.570+12.570area=7,630cm2V