TXIncome Tax Liability Question

Dear Sir, I am doing a question from the BPP revision kit but I am having an issue. Below is the question: Lauren’s father, Tom, a widower, who was born in 1944, has the following income for tax year 2013/14. State retirement pension 4,926 Pension from former employer (before deduction of PAYE) 7,890 Building society interest (net interest credited) 14,320 Tom gave £3,200 to Oxfam (a registered charity) on 21 September 2013 under Gift Aid. This was the actual amount paid. Required Calculate Tom’s income tax liability for tax year 2013/14. Answer: Non-savings Savings Total income income £ £ £ State pension 4,926 Employment pension 7,890 BSI (14,320 × 100/80) 17,900 Net income 12,816 17,900 30,716 Less higher personal allowance (W)(10,192) (10,192) Taxable income 2,624 17,900 20,524 Tax £ On Non-savings income £2,624 @ 20% 525 On Savings income (£2,790 – £2,624) = £166 @ 10% 17 (£17,900 – £166) = £17,734 @ 20% 3,547 Income tax liability 4,089 Working Higher personal allowance £ £ Higher personal allowance born between 6 April 1938 and 5 April 1948 10,500 Net income 30,716 Less Gift Aid £3,200 × 100/80 (4,000) 26,716 Less limit (26,100) 616 ÷2 (308) Revised higher personal allowance 10,192 Regarding the tax calculations can you tell me the reason for multiplying the 10% by 166. I cannot seem to figure out where this figure is coming from. Thanks you.