Rent Payable

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I have a student let & charge £68.00 per person per week, with half summer (July & August) rent concession.
I currently charge each tenant as follows: £68 x 52 = £3536 / 12 = £294.66 pcm.
One of the tenants has now emailed me as below his interpretation of what he should be paying:

I'm in the process of setting up the standing order and have noticed an error. You've set the rent at £68 a week for 52 weeks but shouldn't it be £68 a week for 44 weeks? due to the half price rent concession over the summer months? So the rent should be £68 x 44 plus £34 x 8 which works out at £272 a month not £294.66. I've sent the £24.66 for the summer but it looks like I needn't have done that. So could that be taken out of next months instalment? If any of that is wrong and I've missed something, please let me know and I'll adjust accordingly.

How can I explain to him in simple terms that he's incorrect??