Group COY kaplan exam kit qn 250

Hello sir had a qn

Telephone Ltd prepares accounts to 31 March each year. Throughout the year ended
31 March 2022 Telephone Ltd owned 62% of Desk Ltd, 75% of Chair Ltd, 55% of Table Ltd (a
dormant company) and 100% of Window Inc (resident overseas). Telephone Ltd acquired
60% of the share capital of Curtain Ltd on 1 January 2021.
What is the total number of companies for the purposes of adjusting the £1,500,000
augmented profits threshold of Telephone Ltd for the year ended 31 March 2022?

ANS
The answer in the kit is 4 companies, whereby curtain ltd is excluded from the group.
I understand that the number of 51% in a group in an accounting period is determined at the end of the previous AP, and so that is why curtain ltd was not included since it wasn’t part of the previous AP 51% group.

However we also know that if a coy joins a 51% group coy in the current AP, it is deemed to be part of the group from the beginning of the following AP.

And that is just the case here as well!
Curtain joined the group in the last AP, therefore it wasn’t a part of the previous AP but will be a part of this AP 51% group basing on the explanation above.

Then why is it not included as part of the group.
Isn’t the answer supposed to be 5?
am i missing on something or is there a typing error in the qn whereby the date curtain ltd was acquired was supposed to be 1st Jan 2022, not 2021?

Thank you.