200. Modify the formula provided in the previous question so that it allows for the quick calculation of the square of an integer ending in 5. (Use the fact $\frac{1}{2}=0.5$ ).
200. 
$$
\quad(10 a+5)^{2}=100 a(a+1)+25
$$

Rule: multiply the part of the number representing the tens by the next natural number and append 25.