(a)[ the tax which would be chargeable on that part of the relevant income which is applicable to charitable or religious purposes (as reduced by the income, if any, which is exempt under section 11) as if such part (or such part as so reduced) were the total income of an association of persons; and [Substituted by Act 19 of 1970, Section 21, for Section 164 (w.e.f. 1.4.1971).]