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James from Evenlode sent a very well-explained solution this problem. He said:To make pairs to $10$ you need to leave out $5$ and $10$, because you would need a double for the $5$ and a $0$ for the $10$.
Thank you also to Johannes, Fred and Lok from St Barnabas, Henry from Moulsford, children from Jebel Ali Primary School, Alex from Templars Primary, Alex from Maidstone and Beth from Torpoint who all sent clear solutions.