Additional income-tax calculation requires aggregating disclosed income with returns and computing tax on the aggregate, then subtracting prior tax.

The additional amount of tax is calculated differently for single-year and multi-year disclosures. For a single year with a filed return, disclosed income is aggregated with returned total income, tax is computed on the aggregate, and the additional tax equals the tax on the aggregate minus tax on the returned total income; if no return was filed, tax is computed on disclosed income alone. For disclosures across multiple years, additional tax is calculated for each year by the single-year method and then aggregated.

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