Question Number 3824 by Yozzii last updated on 21/Dec/15
$${Box}\:{I}\:{has}\:\mathrm{3}\:{red}\:{and}\:\mathrm{5}\:{white}\:{balls}, \\ $$$${while}\:{Box}\:{II}\:{contains}\:\mathrm{4}\:{red}\:{and}\:\mathrm{2}\: \\ $$$${white}\:{balls}.\:{A}\:{ball}\:{is}\:{chosen}\:{at}\:{random} \\ $$$${from}\:{the}\:{first}\:{box}\:{and}\:{placed}\:{in}\:{the} \\ $$$${second}\:{box}\:{without}\:{observing}\:{its}\:{colour}. \\ $$$${Then}\:{a}\:{ball}\:{is}\:{drawn}\:{from}\:{the}\:{second} \\ $$$${box}.\:{Find}\:{the}\:{probability}\:{that}\:{it}\:{is}\:{white}. \\ $$
Commented by Rasheed Soomro last updated on 21/Dec/15
$${Act}−{I}\:\::{Drawing}\:{a}\:{ball}\:{from}\:{Box}\:{I} \\ $$$${Total}\:{balls}=\mathrm{8},{out}\:{of}\:{which}\:{are}\:\mathrm{5}\:{white} \\ $$$${balls}. \\ $$$${probability}\:{of}\:{being}\:{white}\:{ball}\:{is}\:\frac{\mathrm{5}}{\mathrm{8}} \\ $$$${Act}−{II}\:\::\mathrm{4}\:{red}\:\mathrm{2}\:{white}\:{one}\:\overset{\frac{\mathrm{5}}{\mathrm{8}}} {{white}}/\overset{\frac{\mathrm{3}}{\mathrm{8}}} {{red}} \\ $$$${One}\:{added}\:{ball}\:{with}\:{respect}\:{to}\:{probability} \\ $$$${is}\:\frac{\mathrm{5}}{\mathrm{8}}\:{parts}\:{white}\:{and}\:\frac{\mathrm{3}}{\mathrm{8}}\:{parts}\:{red} \\ $$$${Total}\:{balls}\:{are}\:\mathrm{7} \\ $$$${Out}\:{of}\:{which}\:\mathrm{2}+\frac{\mathrm{5}}{\mathrm{8}}=\frac{\mathrm{21}}{\mathrm{8}}\:{white}\:{balls} \\ $$$${probability}\:{being}\:{white}=\frac{\frac{\mathrm{21}}{\mathrm{8}}}{\mathrm{7}}=\frac{\mathrm{21}}{\mathrm{8}×\mathrm{7}}=\frac{\mathrm{3}}{\mathrm{8}} \\ $$
Commented by prakash jain last updated on 21/Dec/15
$${E}_{\mathrm{1}} =\mathrm{White}\:\mathrm{ball}\:\mathrm{drawn}\:\mathrm{from}\:\mathrm{Box1} \\ $$$${E}_{\mathrm{2}} =\mathrm{Red}\:\mathrm{ball}\:\mathrm{drawn}\:\mathrm{from}\:\mathrm{Box1} \\ $$$${A}=\mathrm{getting}\:\mathrm{white}\:\mathrm{ball}\:\mathrm{from}\:\mathrm{Box2} \\ $$$$\mathrm{P}\left(\mathrm{A}\right)=\mathrm{P}\left(\mathrm{A}\mid\mathrm{E}_{\mathrm{1}} \right)\mathrm{P}\left(\mathrm{E}_{\mathrm{1}} \right)+\mathrm{P}\left(\mathrm{A}\mid\mathrm{E}_{\mathrm{2}} \right)\mathrm{P}\left(\mathrm{E}_{\mathrm{2}} \right) \\ $$$$\mathrm{P}\left(\mathrm{E}_{\mathrm{1}} \right)=\mathrm{5}/\mathrm{8} \\ $$$$\mathrm{P}\left(\mathrm{E}_{\mathrm{2}} \right)=\mathrm{3}/\mathrm{8} \\ $$$$\mathrm{P}\left(\mathrm{A}\mid\mathrm{E}_{\mathrm{1}} \right)=\mathrm{3}/\mathrm{7} \\ $$$$\mathrm{P}\left(\mathrm{A}\mid\mathrm{E}_{\mathrm{2}} \right)=\mathrm{2}/\mathrm{7} \\ $$$$=\frac{\mathrm{3}}{\mathrm{7}}×\frac{\mathrm{5}}{\mathrm{8}}+\frac{\mathrm{2}}{\mathrm{7}}×\frac{\mathrm{3}}{\mathrm{8}}=\frac{\mathrm{15}+\mathrm{6}}{\mathrm{56}}=\frac{\mathrm{21}}{\mathrm{56}}=\frac{\mathrm{3}}{\mathrm{8}} \\ $$
Commented by Yozzii last updated on 21/Dec/15
$${Thanks} \\ $$