Non-Adjacent Ones

Combinatorics
Binary Strings
Medium
How many binary strings of length 10 contain exactly four 1s with no two 1s adjacent?
Hint 1

Arrange the 6 zeros first, creating gaps to place the ones.

Hint 2

Placing 6 zeros creates 7 gaps; choose 4 gaps for the ones.

Show worked solution
  1. 1. Set up gap method

    Place the 6 zeros in a row, creating 7 gaps (including ends) where ones can go, at most one 1 per gap to avoid adjacency.

  2. 2. Choose gaps

    Choose 4 of these 7 gaps for the ones: .

Answer: 35

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