# Codercise I.13.5

**URL:** <https://discuss.pennylane.ai/t/codercise-i-13-5/8519>\
**Category:** Codebook\
**Created:** [May 17, 2025, 7:11pm UTC](https://discuss.pennylane.ai/t/codercise-i-13-5/8519 "2025-05-17T19:11:37Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![pavitraallamaraju](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/pavitraallamaraju/32/3783_2.png) [@pavitraallamaraju](https://discuss.pennylane.ai/u/pavitraallamaraju)\
**Post date:** [May 17, 2025, 7:11pm UTC](https://discuss.pennylane.ai/t/codercise-i-13-5/8519/1 "2025-05-17T19:11:37Z")

</div>

```auto
# Wires 0, 1, 2 are the control qubits
# Wire 3 is the auxiliary qubit
# Wire 4 is the target
dev = qml.device("default.qubit", wires=5)

@qml.qnode(dev)
def four_qubit_mcx_only_tofs():
    # We will initialize the control qubits in state |1> so you can see
    # how the output state gets changed.
    qml.PauliX(wires=0)
    qml.PauliX(wires=1)
    qml.PauliX(wires=2)

    ##################
    # YOUR CODE HERE #
    ##################

    # IMPLEMENT A 3-CONTROLLED NOT WITH TOFFOLIS

    qml.Toffoli(wires=[0, 1, 3])
    qml.Toffoli(wires=[3, 2, 4])
    qml.Toffoli(wires=[0, 1, 3])
    return qml.state()

print(four_qubit_mcx_only_tofs())

```

Can’t figure out what’s going wrong here. It keeps giving me this error- Error: Your gates are not applied to the correct wires.

---

<div class="post-metadata">

**Author:** ![pavitraallamaraju](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/pavitraallamaraju/32/3783_2.png) [@pavitraallamaraju](https://discuss.pennylane.ai/u/pavitraallamaraju)\
**Post date:** [May 17, 2025, 7:30pm UTC](https://discuss.pennylane.ai/t/codercise-i-13-5/8519/2 "2025-05-17T19:30:44Z")

</div>

It works when I changed the second Toffoli to [2,3,4].

---

<div class="post-metadata">

**Author:** ![CatalinaAlbornoz](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/catalinaalbornoz/32/1196_2.png) [@CatalinaAlbornoz](https://discuss.pennylane.ai/u/CatalinaAlbornoz)\
**Post date:** [May 20, 2025, 4:21pm UTC](https://discuss.pennylane.ai/t/codercise-i-13-5/8519/3 "2025-05-20T16:21:28Z")

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Hi @pavitraallamaraju ,

Welcome to the Forum!

It’s great to see that you solved the codercise. I’ve blurred your answer to avoid spoilers for others.

I hope you keep enjoying the Codebook!