The arithmetic mean and standard deviation of series of 20 items were calculated by a student as 20 cm and 5 cm respectively. But while calculating them an item 15 was misread as 30. Find the correct standard deviation.

Given the marks of 25 students in the class as $\{m_1,m_2,m_3,..m_{25}\}$. Marks lie in the range of [1-100] and $\overline{m}$ is the mean. Which of the following quantity has the value zero?