## 2 Answers

Best answer

The answer is 20.

See below image

**OR**

One way to select root.

10

/ \

**LST (3 elements) RST (2 elements)**

**Now LST contains an element out of 5 (As the root is chosen)**

**So Total ways to choose element $_{}^{5}\textrm{C}_{3}$ **

**and RST contains 2 element So number of ways = $_{}^{2}\textrm{C}_{2}$ **

**Let LST chosen { 20,30,40} **

**Now 2 chosen as root of LST (in one way)**

**Now remaining 2 element can arrange in 2! **

**ie. 20 20**

** / \ / \**

** 30 40 40 30**

**Therefore Total Possibility = ****$_{}^{5}\textrm{C}_{3}$ ** **x** **2! x $_{}^{2}\textrm{C}_{2}$**

** = 20**