Apologies but the answers to questions 17a a should be computed after taking into account the tax rebates (see page 15 , 3 sentences at bottom of tax bracket table)
tax payable = $366.25
tax payable after rebate = (0.7) x 366.25 = $256.38
because there is a tax rebate (discount) for persons below 60 years of age as stated in table.
final tax payable = $0
because there is a parenthood rebate of $5000 for Mr Heng (as stated in question)