Draw an arithmetic expression tree that has four external nodes, storing the numbers 1, 5, 6, and 7 (with each number stored in a distinct external node, but not necessarily in this order), and has three internal nodes, each storing an operator from the set {+,−,∗,/}, so that the value of the root is 21. The operators may return and act on fractions, and an operator may be used more than once.