How many children are there between A and B in a row of children ?

Statement I : A is seventeenth from the left in the row.

Statement II : B is exactly in the middle and there are ten children towards his right, as well as to his left.

The data in both statements I and II together are necessary to answer the question.

Statement I: A is seventeenth from the left in the row. This tells us that A is 17th out of a total of ... ( unknown) children in the row.

Statement II: B is exactly in the middle and there are ten children towards his right, as well as to his left. This tells us that B is at the middle position (since there are an equal number of children to his right and left). The number of children to his right is 10, so the total number of children in the row must be 2 × 10 + 1 = 21.

Now, we can find the number of children between A and B. Since A is 17th from the left, and B is at the middle (11th position), there are:

11 - 17 = 6 children between A and B.

Thus, "The data in both statements I and II together are necessary to answer the question" is correct.