ClassInterval	Freq(f)	Mid-point(x)	\(fx\)	\(x^{2}\)	\(fx^{2}\)
60 - 64	2	62	124	3844	7688
65 - 69	3	67	201	4489	13487
70 - 74	6	72	432	5184	31104
75 - 79	11	77	847	5929	65219
80 - 84	8	82	656	6724	53792
85 - 89	7	87	609	7569	52983
90 - 94	2	92	184	8464	16928
95 - 99	1	97	97	9409	9409
\(\sum\)	40		3150		250590

(a) Mean \(\bar{x} = \frac{\sum fx}{\sum f}\)
= \(\frac{3150}{40}\)
= 78.75
(b) \(SD = \sqrt{\frac{\sum fx^{2}}{\sum f} - (\frac{\sum fx}{\sum f})^{2}}\)
= \(\sqrt{\frac{250590}{40} - (\frac{3150}{40})^{2}}\)
= \(\sqrt{6264.75 - (78.75)^{2}}\)
= \(\sqrt{6264.75 - 6201.5625}\)
= \(\sqrt{63.1875}\)
= 7.95.