Question

Sol. [Key Concept: Mean $m = \frac{\sum\ f_{i}x_{i}}{\sum\ f_{i}}$, then mean deviation $= \frac{\sum\ f_{i}{|x}_{i} - m|}{\sum\ f_{i}}$.]

M.D. $= \frac{\sum f_{i}\left| x_{i} - \bar{x} \right|}{\sum f_{i}}$

$${\bar{x} = \frac{\sum f_{i}x_{i}}{\sum f_{i}} = \frac{40 + 42 + 18 + 20 + 24 + 90}{26} }{= \frac{234}{26} = 9 }$$M.D. $= \frac{8(4) + 6(2) + 2(0) + 2(1) + 2(3) + 6(6)}{26}$

$$= \frac{84}{26} = \frac{44}{13} $$