Notice that $15$inches$-10$inches is two standard deviations since $\sigma=2.5$ inches. Using the 68-95-99 rule tells us that only $5\%$ of all the trees will lie outside of two standard deviations from the mean. Since we are only looking at being bigger than the mean instead of bigger or smaller, that means we cut that $5\%$ number in half to get $2.5\%$. So, $2.5\%$ of the maple trees will be bigger than $15$inches and since there are $n=2000$ trees, that means we expect $50$ to be bigger than $15$inches.