The first batch takes 10.5 hours, meaning only .5 batches are left. You then take the time that each new full batch takes (4.25) and divide it by 2 as you only need half of one batch. Meaning 0.5 of a batch takes 2.125 hours. You then take the time of the two batches (10.5, and 2.125) and add them, leaving you with 12.625. Meaning 12.375 is the most viable answer.
