INCOME TAX

20 Based on her income tax liability of £15,600 for the tax year 2020/21,Sarah was liable to make two payments on account for the tax year 2021/22 of £7,800each. In May 2022,Sarah made a claim to reduce her second payment on account to £6,000. This reduced payment was made on 30 September2022. Sarah’s actual income tax liability for the tax year 2021/22 was £16,000,and she paid the full balance outstanding on 31January 2023. How much interest is payable by Sarah to HM Revenue &Customs(HMRC) in respect of her second payment on account for the tax year 2021/22?
ANS
42
The most important point was the £6,000 payment on 30 September2022,which
was two months late. The final payment was made on time.Therefore,£7,800 was late for
two months(31July to 30 September2022)and £1,800(£7,800– £6,000)was late for four
months(1October2022 to 31January 2023).The interest at 2.60%is£49,ie(£7,800*
2.60%*2/12)+(£1,800*2.60%*4/12),making a total of £49.An alternative working
would have been £6,000 late for two months and £1,800 late for six months.

Good day, Please i don’t understand how 42 was gotten.I’ll appreciate it if you can explain better.