91Ó°ÊÓ

A letter at random from $RESERVE$ and then chooses one at random from $VERTICAL$.

Experiment: Choose a letter form $\left\{R_1,E_1,S_1,E_2,R_2,V_1,E_3\right\}$and then one from the localid="1649734539307" $\left.\left\{V_1,E_1,R_1,T_1,I_1,C_1,A_1,L_1\right\}\right\}$

Find: the probability that the same letter is chosen.

Outcome space $S$contains every pair of a letter from the first word and a letter from the second word.

If all events in $S$are considered equally likely, probability of event $A⊆S$is:

$P\left(A\right)=\frac{\left|A\right|}{\left|S\right|}$

where $\left|X\right|$denotes the number of elements in $X$.

The number of elements in $S$:

$7$choices for the first letter, and $8$choices for the second letter. - $\left|S\right|=56$

The event $A$- the same letter is chosen from both words.

A is an union of three mutually exclusive events $A=A_R∪A_E∪A_V·A_R,A_E,A_V$are events where two Rs, two Es or two Vs are chosen, respectively.

localid="1650021930191" $$\begin{gathered} A_R=\left\{R_1{R}_1,R_2{R}_1\right\}⊆S,\text{and}\left|A_R\right|=2⇒P\left(A_R\right)=\frac{2}{56}=\frac{1}{28} \\ {A}_E=\left\{E_1{E}_1,E_2{E}_1,E_3{E}_1\right\}⊆S,\text{and}\left|A_E\right|=3⇒P\left(A_E\right)=\frac{3}{56} \\ {A}_V=\left\{V_1{V}_1\right\}⊆S,\text{and}\left|A_V\right|=1⇒P\left(A_V\right)=\frac{1}{56} \end{gathered}$$

Since $A_R,A_E,A_V$are mutually exclusive

$P\left(A\right)=P\left(A_R\right)+P\left(A_E\right)+P\left(A_V\right)=\frac{6}{56}=\frac{3}{28}$