Please reason step by step, and put your final answer within \boxed{}.
Mark initially has 13 trees in his backyard. He decides to remove 3 trees due to their old age. Mark then buys 18 new trees from a nursery, but only plants 12 of them immediately, saving the rest for later. If Mark plants an additional 25% more trees than he initially planned, how many trees will he have in his backyard in total?
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