Answer: A tree with 6 nodes should have 5 edges (the number of edges of the tree is equal to the number of nodes minus 1, that is, e=v- 1).

The total degree of graph G is 18, so graph G has 18/2=9 edges. (The total degree of a graph is twice the number of edges)

Therefore, 4(9-5=4) edges are deleted from G to make it a tree.