Answers

2016-03-04T22:24:33+08:00
Mean = Σ(fx) ÷ Σf

Σ- summation/total
fx- frequency times class mark
f- frequency

So, for each class, multiply it with its class mark  (average of the highest and lowest scores in the class interval) and add the results. You'll get Σ(fx).

For 
Σf, just add all the frequencies.

Example:
Class Interval///Class mark(x)///Frequency(f)///(fx)
92-99/////////////95.5////////////////////////2///////////////191
100-107 ////////103.5/////////////////////7///////////////724.5

z = Σ(fx) ÷ Σf
Σ(fx) = 915.5 [191+724.5]
Σf = 9 (2+7)
Σ(fx) ÷ Σf = 915.5 ÷ 9 = 101.72

Sorry, it's kind of hard to explain online. But it's not as hard as it seems.

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