Correct option is A. $$3.38$$
Computation of mean deviation about mean
$$x_i$$ | $$f_i$$ | $$x_i\ f_i$$ | $$|x_i -9|$$ | $$\displaystyle \sum f_i |x_i -9|$$ |
$$5$$ | $$8$$ | $$40$$ | $$4$$ | $$32$$ |
$$7$$ | $$6$$ | $$42$$ | $$2$$ | $$12$$ |
$$9$$ | $$2$$
| $$18$$ | $$0$$ | $$0$$ |
$$10$$ | $$2$$ | $$20$$ | $$1$$ | $$2$$ |
$$12$$ | $$2$$ | $$24$$ | $$3$$ | $$6$$ |
$$15$$ | $$6$$ | $$90$$ | $$6$$ | $$3$$ |
| $$N=\displaystyle \sum f_i =26$$ | $$\displaystyle \sum f_i x_i =234$$ | | $$=\displaystyle \sum f_i |x_i -9| =88$$ |
We have, $$N=\displaystyle \sum f_i=26$$, and $$\displaystyle \sum f_i\ x_i=234$$
$$\therefore \ $$ Mean $$=\bar X=\dfrac {1}{N}\displaystyle \sum f_i \ x_i =\dfrac {234}{26}=9$$
Mean deviation $$=\dfrac {1}{N}\displaystyle \sum f_i |x_i -9|=\dfrac {234}{26}=9$$
Mean deviation $$=\dfrac {1}{N}=\displaystyle \sum f_i |x_i -9 =|\dfrac {88}{26}=3.38$$