=SUMPRODUCT(((MONTH($A$5:$A$29)=MONTH(G10))*(YEAR($A$5:$A$29)=YEAR(G10))*
($E$5:$E$29)))/SUMPRODUCT(((MONTH($A$5:$A$29)=MONTH(G10))*(YEAR($A$5:$A$29)=YEAR(G10))))