>>211
Wolfram先生に訊いてみた
https://www.wolframalpha.com/input/?i=%28sum+10000-%28500i%2B600j%2B800%2810-i-j%29%29,i%3D0+to+10-j,j%3D0+to+10%29%2F%28sum+1,i%3D0+to+10-j,j%3D0+to+10%29

(sum 10000-(500i+600j+800(10-i-j)),i=0 to 10-j,j=0 to 10)/(sum 1,i=0 to 10-j,j=0 to 10)
結果は11000/3≒3666.67となった．