The calculation for savings is:
£18,600 - annual salary = shortfall
shortfall x 2.5 = shortfall over 2.5 years
shortfall over 2.5 years + 16,000 = amount of savings required.
So if the annual salary was 12,820:
£18,600 - £12,820 = £5,780
£5,780 x 2.5 = £14,450
£14,450 + £16,000 = £30,450