bpp question

Dear Sir, Could you please explain me this question on self assessment 20 Based on her income tax liability of £15,600 for the tax year 2018/19, Sarah was liable to make two payments on account for the tax year 2019/20 of £7,800 each. In May 2020, Sarah made a claim to reduce her second payment on account to £6,000. This reduced payment was made on 30 September 2020. Sarah's actual income tax liability for the tax year 2019/20 was £16,000, and she paid the full balance outstanding on 31 January 2020. How much interest is payable by Sarah to HM Revenue & Customs (HMRC) in respect of her second payment on account for the tax year 2019/20? ? £61 ? £42 ? £29 ? £32 did not understand the answer