Mean = Σ(fx) ÷ Σf
fx- frequency times class mark
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.
Class Interval///Class mark(x)///Frequency(f)///(fx)
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.