Seven cakes cost £2.10 how much do 10 cakes cost

Ten cakes costs $2.19 (pounds, but I don't have a pounds symbol on my computer) because if you divide $2.10 by seven you get $0.3, and if you add 3×0.3 (because we have seven, and we need to add three more to get ten) we get $0.9.
 
0.9+2.10=2.19