Effective units for conversion costs: 1600 are full complete, WIP is 50% complete so effective units – 400 x 50% = 200. Total effective units = 1600 + 200 = 1800.
So cost per effective unit = 12240 / 1800 = 6.80.
Value of WIP: materials 400 x $4; plus conversion costs 400 x 50% = $6.80