The probability formula is given by:
![\text{ P(E) =}\frac{\text{ N(Required outcome)}}{\text{ N(Total outcome)}}](https://img.qammunity.org/2023/formulas/mathematics/college/ijpq6c3ilmhpi0wywubmro6ci1s5m72o2g.png)
The person selects one of six envelopes.
There is a probability that the person selects an envelope that contains a $491 of 3 envelopes OR an envelope that contains a $1003 check of 3 envelopes
![\begin{gathered} \text{ P(select one containing \$491 check) =}(3)/(6)=(1)/(2) \\ \text{ P(select one containing \$1003 check) =}(3)/(6)=(1)/(2) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/1i07viufegqf0ktsqgjyjxpqzikxe6999n.png)
Then, the expected value is given by
![\begin{gathered} \text{ P(Expected Value) =(}(1)/(2)*491)\text{ +(}(1)/(2)*1003) \\ =245.5+501.5 \\ =747.0 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/q9cdl78gro8jvt7ir2wca25btbhoe39l3z.png)