We are given that the total cost of the holiday was $4347.In the family there were 2 adults & 3 children and the cost for 1 adult was double the cost for 1 child.
Now let the cost for one child be C. So the cost for an adult is 2C.
=> 2*2C + 3C = 4347
=> 4C + 3C = 4347
=> 7C = 4347
=> C = 4347/7
=> C = 621.
Therefore the cost for one child was $621.