(a) Taxable income = \(N150,000 - N40,000 = N110,000\)
Tax payment = \(N110,000 \times 0.25 = N27,500\)
Income left = \(N(150,000 - 27,500) = N122,500\)
(b) \(% gain = \frac{\text{actual profit}}{\text{cost price}} \times 100%\)
= \(\frac{SP - CP}{CP} \times 100%\)
\(\therefore \frac{75 - CP}{CP} = \frac{30}{100} \implies 0.3CP = 75 - CP\)
\(1.3CP = 75 \implies CP = N57.69\)
Total cost price = \(650 \times \frac{750}{13} = N37,500\)
350 copies sold at N75 ==> \(SP = 350 \times N75 = N26,250\)
300 copies sold at 5% discouunt ==> \((75 - \frac{5}{100} \times 75) \times 300\)
= \((75 - 3.75) \times 300 = N21,375\)
Total selling price = \(N(26,250 + 21,375) = N47, 625\)
Profit = \(N(47,625 - 37,500) = N10,125\)
\(% = \frac{10,125}{37,500} \times 100% = 27%\)