Question

A bag contains white, black and red balls only. A ball is drawn random from the bag. If the probability of getting a white ball is $\frac{3}{10}$ and that of a black ball is $\frac{2}{5}$, then find the probability of getting a red ball. If the bag contain $20$ black balls, then find the total number of ball is in the bag.

Answer 1

P (of red ball) $=1-(\frac{3}{10}+\frac{2}{5})$

$=1-(3+\frac{4}{10})$

$=1-\frac{7}{10}=\frac{10-7}{10}$

P (of red ball) $=\frac{3}{10}$

No. of black balls $=\frac{20}{totalno.ofballs}$

$\frac{2}{5}=\frac{20}{totalno.ofballs}$

$\Rightarrow 50$ = total no. of balls