The probability of selecting a green marble at random from a jar that contains only green, white and yellow marbles is 1/4. The probability of selecting a white marble at random from the same jar is 1/3. If this jar contains 10 yellow marbles. What is the total number of marbles in the jar?
GIVEN: A bag contains green, white and yellow marbles.
(i) Probability of selecting green marbles = 
(ii) Probability of selecting white marbles = 
(iii) The jar contains 10 yellow marbles.
TO FIND: Total number of marbles in the same jar
We know that sum of probabilities of all elementary events is 1.
Hence,
$\mathrm{P}($ green marble $)+\mathrm{P}$ (white marble) $+\mathrm{P}$ (yellow marble) $=1$
$\frac{1}{4}+\frac{1}{3}+P($ yellow marble $)=1$
$\frac{3+4}{12}+\mathrm{P}($ yellow marble $)=1$
$\frac{7}{12}+\mathrm{P}($ yellow marble $)=1$
$\mathrm{P}($ yellow marble $)=1-\frac{7}{12}$
$\mathrm{P}($ yellow marble $)=\frac{5}{12}$
We know that PROBABILITY $=\frac{\text { Number of favourable event }}{\text { Total number of event }}$
Hence
$\frac{5}{12}=\frac{\text { Number of favourable event ie yellow marble }}{\text { Total number of marble }}$
$\frac{5}{12}=\frac{10}{\text { Total number of marbles }}$
Total number of marbles $=\frac{10 \times 12}{5}$
Total number of marbles $=24$