1/3 x 5.70 = £1.90 (what a grandparent payed)
£1.90 x 2 = £3.80 (the cost for both grandparents)
5.70/2 = £2.85 (what one child payed)
2.85 x 3 = £8.55 (the cost for all three children)
5.70 x 2 = £11.40 (the cost for both parents)
Add them all up
3.80 + 8.55 + 11.40
= £23.75 for the whole family