help plz dont understand answer

Jano operates the annual acounting scheme.In the year ended 30 september 2014 he had a VAT liability of £3600 .In his year ended 30 september 2015 he estimated his VAT liability to be £4115
What is the amount of his balancing payment for the VAT year to 30 september 2015?

there will have been 9 payments at 10% of last year’s liability in months 4 to 12 totalling £3240 (9*10%*£3600)
this year’s liability is £4115 leaving a balance left to pay of £875(£4115-£3240)