6 Determine the personal tax rate on interest income at which the tax advantage of debt is zero. (15 marks)

Biller Industries plc is a global haulage equipment and scaffolding manufacturer. The company has never borrowed before but feels that, in order to maximize growth and increase value, a debt issue is required.
Currently the firm has 50 million shares outstanding with a share price of £1.50. The profit before taxes is forecast to be £25 million. Biller Industries requires £30 million to fund its expansion plans. The firm feels that it could borrow £45 million and use the additional £15 million to also buy back shares in the company.
The corporate tax rate is 18 per cent.