If the cost of 2kg of carrots is £7, to find 4kg we need to double this cost:
£7 x 2 = £14
4 kg = £14
To find the cost of 5 kg of carrots we must add another 1 kg, we can do this by halving £7 to find 1 kg and then adding the cost of 4 kg (£14) onto it:
£7 ÷ 2 = £3.50
£14 + £3.50 = £17.50
This means the cost of 5 kg of carrots will be £17.50