Another way would be to use a conditional array...say you have values
in range a1:a12, you would enter the summation formula in a13 as follows:
=SUM(IF(MOD(A1:A12,2)=0,A1:A12))
to get the formula to work, you MUST hold down the <Ctrl><Shift> then
<Enter>. The result formula would have two brackets around the
formula like below...

{=SUM(IF(MOD(A1:A12,2)=0,A1:A12))}

the result should be 30