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Ten unit steps from 0 ending at 4

Question

Count the sequences x0,x1,…,x10x_0,x_1,\dots,x_{10} of numbers such that x0=0x_0=0, x10=4x_{10}=4, and each consecutive pair differs by exactly one, that is ∣xi−xi−1∣=1|x_i-x_{i-1}|=1 for i=1,…,10i=1,\dots,10.

Short answer

120120. Each step is +1+1 or −1-1; to net +4+4 in 1010 steps you need 77 up-steps and 33 down-steps. Choosing where the down-steps go gives (103)=120\binom{10}{3}=120.

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