Income 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.