Since the state of the first qubit is different in these states ($\ket +$ and $\ket -$, respectively), it is sufficient to measure only the first qubit in the Pauli X basis in order to distinguish the two states. Furthermore, this measurement will not change the state of the system, since the post-projection state is identical to the original state regardless of the input state.

@[solution]({
    "id": "multi_qubit_measurements__partial_measurements_for_system_solution",
    "codePath": "Solution.qs"
})
