Input Tax Credit allocation and reversal required where credit is partly used for exempt or non business purposes. Rule 42 prescribes variables T, T1, T2, T3, T4 and C1 (C1 = T - (T1+T2+T3)) to attribute input tax credit. After identifying T4, common credit C2 = C1 - T4 is apportioned: D1 = (E/F) x C2 for exempt supplies and D2 = 5% of C2 for non business use. Final eligible credit C3 = C2 - (D1 + D2). C3, D1 and D2 are computed separately for each tax component and declared; the aggregate D1+D2 is reversed via return or adjustment form.