It was not enough for Tenhos Sacco to apportion the expenses to BOSA and FOSA. It should have gone further and determined which income under BOSA is subject to tax and which income is exempt. The Tribunal further found that by failing to provide a further breakdown of its income based on whether it was exempt or chargeable to tax and apportion the expenses accordingly, Tenhos Sacco had failed to discharge the burden of proving that a tax decision is wrong.