Exchange rate variation adjustments must modify asset cost and income computation to reflect altered rupee liabilities. Where exchange rate movements after acquisition of an asset bought from abroad alter the rupee liability for payment of the asset's cost or repayment of a foreign-currency loan taken to acquire the asset, the change in liability during the previous year must be added to or deducted from the asset's actual cost; assessing officers must use notified revised exchange rates when computing income to ensure correct taxation.