Marks obtained by 30 students of a Patna coaching batch:
30-40: 4, 40-50: 6, 50-60: 10, 60-70: 8, 70-80: 2.
Kunal chose the assumed mean $a=45$ and correctly found $\sum f_i{d}_i=280$. What mean does he obtain?

Mean $=a+\frac{\sum f_i{d}_i}{\sum f_i}=45+\frac{280}{30}=45+9.33=54.33$. Any other assumed mean gives the same value — with $a=55,\sum f_i{d}_i=-20$ and 55 - 0.67 = 54.33 as well. "9.33" reports only the correction term and forgets to add a; "325" adds 280 to 45 without dividing by $\sum f_i$; "53" comes from mis-adding the frequencies as 35 instead of 30.