Question

how to calculate mode if two classes have same and highest frequency (bimodal) ?

Answer 1

If there are more than one class intervals which have the same frequency (equally qualifying to be the mode class) then both of the classes will be the mode class. this is called bimodal.
However to calculate the mode of grouped data use the following formula
Mode = L + [ (F - F1) / { (F - F1) + (F - F2) } ] * h
where
L = Lower limit of the modal class
F = Frequency of the modal class
F1 = Frequency of the class immediately previous of modal class
F2 = Frequency of the class immediate next of modal class
h = Range of the modal class (higher limit - lower limit)