Question

An urn contains 25 balls of which 10 balls bear a mark X and the remaining 15 bear a mark Y. A ball is drawn at random from the urn, its mark note down and it is replaced. If 6 balls are drawn in this way, find the probability that

not more than 2 will bear Y mark

Answer 1

It is case of Bernoulli trials with n=6. Let success be defined as drawing a ball marked X.

p=P(a success in a single draw)=$\frac{10}{25}=\frac{2}{5}$

$\Rightarrow q=1-\frac{2}{5}=\frac{3}{5}$

Clearly, Z has a binomial distribution with n=6, $p=\frac{2}{5}andq=\frac{3}{5}$

$\therefore$ $P(Z=r){=}^n{C}_r{p}^r{q}^{n-r}{=}^6{C}_r{\left(\frac{2}{5}\right)}^r{\left(\frac{3}{5}\right)}^{6-r}$

P(not more than 2 bear mark Y)

=P(not less than 4 bear mark X)= P(atleast 4 successes)

$P\left(4\right)+P\left(5\right)+P\left(6\right){=}^6{C}_4{p}^4{q}^2{+}^6{C}_5{p}^{5}q{+}^6{C}_6{p}^6{q}^0$

${=}^6{C}_4{\left(\frac{2}{5}\right)}^4{\left(\frac{3}{5}\right)}^2{+}^6{C}_5{\left(\frac{2}{5}\right)}^5{\left(\frac{3}{5}\right)}^1{+}^6{C}_6{\left(\frac{2}{5}\right)}^6{\left(\frac{3}{5}\right)}^0=15\times {\left(\frac{2}{5}\right)}^4\frac{9}{25}+6\times {\left(\frac{2}{5}\right)}^5\frac{3}{5}+1\times {\left(\frac{2}{5}\right)}^6\times 1={\left(\frac{2}{5}\right)}^4[\frac{27}{5}+\frac{36}{25}+\frac{4}{25}]={\left(\frac{2}{5}\right)}^4\left[\frac{135+36+4}{25}\right]={\left(\frac{2}{5}\right)}^4\times \frac{175}{25}=7{\left(\frac{2}{5}\right)}^4$