Minecraft wiki entry for Before_1.8, associated text: Enchanting a newly crafted Bow with an Infinity I Book: the only cost is 1 * cost-per-level * 3/4 = 8 * 3/4 = 6. No rounding was needed in any steps, so that result is exact.
Adding Infinity I to a Bow already enchanted with Power III and Flame I, with Prior Work = 6: Original Base Value is 3*1 + 1*4 + (3*2/2) = 10, and the Modified Base Value is 3*1 + 1*(4/2) + (3*2/2) = 8, 2 enchants, and halved cost-per-level is 1*8/2 = 4. So (8+6+2+4)/2 + 4 = 14 levels. If Infinity I was obtained from another bow instead of a book it would cost 35 levels, 21 more.
A Sharpness 4 sword (BV 5, MBV also 5) for the target, and a Looting 2 Book for the sacrifice. Looting costs 4 (per level) × 2 (levels) / 2 (half cost) → 4×2/2→4, and target already has 1 enchantment, so (5+1+4)/2 + 4 → 9 (rather than 23 from item+item sacrifice)