payment to HMRC

Dear Tutor, would you please help me about this question.
Gareth’s tax payment for 2014/15 and 2015/16 is as follows:
…………………………………………………………..2014/15…………………….2015/16
Income tax ……………………………………………10,000 …………………….12,500
Class 2 NIC ………………………………………………143………………………….146
Class 4 NIC…………………………………………… 2,000……………………….2,500
Capital gains tax …………………………………….1,000………………………..2,000
Gareth always pays the correct amout of tax on each due date.
what is the amout payable by Gareth to HMRC on 31 January 2017 in respect of the tax year 2015/16?
the answer is 5146, but based on the note , there should be another payment which is payable to HMRC, (12,500+2,500)/2=7,500.
please help me.
many thanks