Interest for defaults in furnishing return of income imposes simple monthly interest where returns are filed late or not filed. Interest for defaults in furnishing return of income requires simple monthly interest on specified unpaid tax when returns are late or not filed, with the statute specifying start and end dates and the tax base for distinct filing and notice-driven circumstances, and providing for adjustment of interest following orders that alter the tax base and for reductions by interest already paid and specified definitions of tax paid.