It's 15% tax on interest.
PWC:
"Interest received from bank deposits, loans to finance companies, debentures, and bills issued by a corporate entity is subject to WHT at a flat rate of 15%. Individuals may choose to exclude interest income from other income, in which case they pay the 15% WHT, or they may choose to include such interest income with other income and pay tax according to the PIT rates, in which case the tax withheld at source is credited against the tax liability."