Good question!
If basic rate tax is 20% on your salary, for each £100 you make, you pay £20 in tax.
In order to receive this basic rate back again, you need to receive 25% of your contribution.
25% of £80 is £20, so this is how much we will claim from HMRC!
For more on pension tax relief, check out this page we have put together to explain it all!