Taxable business income computation requires deduction of business expenses and maintenance of books for income determination. Computation of taxable business income under the normal provisions requires determining turnover or gross receipts and deducting expenses incurred in relation to earning that income; the resulting amount is taxable business income. Taxpayers not using the presumptive scheme must maintain books of account, and income is computed on the basis of information revealed in those books, making proper accounting records and documentary support essential to the regular tax computation.