First, it is divided into two categories: 1, taking the straight road from the island to Monkey Mountain; 2. Bypass from Island Island to Monkey Mountain; Look at two kinds. There are several subclasses below.

In the first category, there are four cases of taking a straight road from the island to the Monkey Mountain: there are two straight and curved roads from the Monkey Mountain to the Elephant Hall; There are also straight roads and detours from Elephant Pavilion to Shihu Mountain, so there are 2x2=4 roads from Monkey Mountain to Shihu Mountain. Similarly, if the second category detours from the island to Monkey Mountain, there are four situations behind it. So 2x4=8 kinds.

Extended data

Combination is one of the important concepts in mathematics. Take out m different elements from n different elements at a time? Synthesizing a group without considering the order is called choosing a combination of m elements from n elements without repetition. The number of species in all such combinations is called the combination number.

Combined nature:

1, complementarity

That is, the number of combinations of m elements from n different elements = the number of combinations of (n-m) elements from n different elements; ?

This property is easy to understand, such as c (9,2) = c (9,7), that is, the method of selecting two elements from nine elements is equal to the method of selecting seven elements from nine elements. ?

Provisions: c (n, 0) = 1 c (n, n) = 1 c (0, 0) = 1?

2. Combinatorial identities?

If m item is selected from n items, the following formula exists: c (n, m) = c (n, n-m) = c (n- 1, m- 1)+c (n- 1, m).

References:

Baidu encyclopedia-combination number