計算總合 頭尾 除 2 (10234)

A4{=SUM((C2:C14/TEXT(COUNTIF(A2:B2,B2:B14),"2;;1"))*(B2:B14<=A2)/MONTH(A2))

另解{=(SUM(C2:INDEX(C:C,MATCH(A2,B:B,)))-(C2+INDEX(C:C,MATCH(A2,B:B,)))/2)/MONTH(A2)