The following table shows the daily wages (in ₹) of 60 workers in a factory:
Class interval
30–40
40–50
50–60
60–70
70–80
Frequency
8
12
20
15
5
(i) Calculate the mean wage by treating the class marks directly.
(ii) Re-calculate the mean wage after first subtracting 60 from every class mark before multiplying with its frequency, and state whether the mean obtained differs from part (i).