What is the in-order successor of 15 in the given binary search tree? The tree…

What is the in-order successor of 15 in the given binary search tree? The tree has root 15, with left child 6 and right child 18. Node 6 has children 3 and 7; node 3 has children 2 and 4; node 7 has right child 13; node 13 has left child 9; and node 18 has children 17 and 20.