Simple interest = Cost Price + (Interest Percentage of Cost Price × number of years or months we are paying off)

a) SI = £20 000 + (5% of £20 000 × 4)
SI = £20 000 + (£1000 × 4)
SI = £20 000 + £4000 = £24 000

b) SI = £20 000 + (5% of £20 000 × 3)
SI = £20 000 + (£1000 × 3)
SI = £20 000 + £3000 = £23 000

£24 000 – £23 000 = £1000 that you saved!