accruals and prepayments (electricity account)

Hello sir,

The electricity account for the year ended 30 June 20X1 was as follows.
$
Opening balance for electricity accrued at 1 July 20X0 – 300
Payments made during the year
1 August 20X0 for three months to 31 July 20X0 – 600
1 November 20X0 for three months to 31 October 20X0 -720
1 February 20X1 for three months to 31 January 20X1 -900
30 June 20X1 for three months to 30 April 20X1- 840
1 August 20X1 for three months to 31 July 20X1- 840

What was the expense charged to the statement of profit or loss for the year ended
30 June 20X1 and the accrual at 30 June 20X1?

Accrued at June 20X1 Charged to SPL
year ended 30 June 20X1
$ $

A $Nil $3,060
B $460 $3,320
C $560 $3,320
D $560 $3,420

I am confused with this question.Here it is your answer that i have copied from other forum topic:
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We need the total expense for the period 1 July X0 to 30 June X1.
So go through the payments and list what is relevant for this period.

The first payment is for May, June, July X0, so only July is relevant – 1/3 x 600 = 200
The whole of the next free payments are relevant: 720 ; 900; 840.

However the last payment is for May, June and July X1. Only May and June are relevant for our period, so – 2/3 x 840 = 560

Then add them all up

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But the right answer for the charge to SPL for the year ended 30 June 20X1 is 3320 in the BPP F3 ACCA revision kit 2016-2017.How can it be? Thanks in advance