ASK ACCA TUTOR – FINANCIAL REPORTING (FR) QUESTION clarification

This question is on unrealised profit in non-current asset…

QUESTION
Paint acquired 90% of Saint on 1 July 20X8. On that date, Paint sold an item of plant to Saint for $2.4million. The plant had cost Paint $2million on 1 January 20X7. Saint has charged $120,000 on this plant since it was acquired, based on a useful life of 5 years.

Required:
What adjustment is required in respect of the plant when preparing the consolidated statement of financial position at 31 December 20X8?

ANSWER(from the book)
• $1,080,000 should be deducted from the carrying amount of plant in Saint’s statement of financial position before adding across.

• $1,080,000 should be deducted from group retained earnings in working 5.

Working 5

$000
Carrying amount in Saint’s accounts at 31.12.X8 ($2.4m – $120k) 2,280
Carrying amount in Paint’s accounts had no transfer occurred
$2m × 3/5 1,200
PUP 1,080

My question is:
-Can I try and calculate that $120,000 charge(as stated in the question)to see how they arrive to that figure? Because I have seen an example exactly like this but diff. figures where they calculate the charge amount even tho it was already in the question. I have tried to work it out myself and I have arrived at that $120,000 but the working does not seem right. If you can work it out it will be great to see how you can do it.

-How did they get 3/5 when calculating Paint’s account if no transfer occurred?Cause as far as I can see it’s more like “1-2” years.

-Am I doing something wrong or not following along or is this an error in the books? This have been bugging me and I can’t seem to move pass it unless I have proper understanding, even if it’s not suppose to be that difficult of a question

Thank you in advance.