LONDON — If you're going to put up a poster about energy saving in an office full of maths whizzes, you'd better be damn sure your calculations are correct.

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Earlier in the week, Twitter user Niall shared the following pictures of his office noticeboard.

A while ago energy saving posters went up in the office. Now someone has responded, and I like it very much. pic.twitter.com/mx5iTeHSWn — Niall (@Notinaboyband) October 31, 2016

The poster on the left is from Carbon Trust, an energy saving reminder to turn off the photocopier overnight (apparently it "wastes enough energy to make 30 cups of tea").

The note on the right, meanwhile, is titled "Analysis of Photocopier energy saving". From the looks of it, someone in the office has taken the time to calculate exactly how many cups of tea leaving the photocopier on overnight actually uses.

You've got to admire the effort this person has made.

According to the calculations, leaving the photocopier on overnight would actually only use up enough energy to make half a cup of tea, not 30.

Meanwhile on Twitter, someone tagged the Carbon Trust to see what they had to say for themselves.

If you follow the link you can see the calculations used (basically, the photocopier in the example is much less energy efficient than the model our mystery note-writer was going by).

Either way, Twitter seemed to enjoy the whole exchange.

@Notinaboyband @squawkbox bloody experts with your facts and figures, ruining a perfectly catchy poster 😂 — Hugh (@ehu) October 31, 2016

@Notinaboyband @dairymanNZ Maybe 30 teeny tiny cups? Metric for a cup of tea that a photocopier drinks could be much smaller than 300mL? — Jacqui (@jacquikiwi) October 31, 2016

Good work, anonymous note writer.