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3. A safe is locked by a combination of of four binary digits (that is, 0 or 1), but theowner has forgotten the combination. The safe is designed in such a way that nomatter how many digits have been pressed, if the correct combination of three digitsis pressed at any point, then the safe automatically opens (there is no "enter" key).Our goal is to find the minimum number of digits that one needs to key in in order toguarantee that the safe opens. In other words, we wish to find the smallest possiblelength of a binary sequence containing every four-digit sequence in it. (a) Create a digraph whose vertex set consists of three-digit binary sequences. From each vertex labelled xyz, there is one outgoing edge (labelled 0) leading to vertexyz0, and another outgoing edge (labelled 1) leading to vertex yzl. (b) Explain why every edge represents a four digit sequence and why an Eulerian tour of this graph represents the desired sequence of keystrokes. (c) Find the minimum number of digits that one needs to key in to guarantee thatthe safe opens.

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