The probability formula is given by:
[tex]\text{ P(E) =}\frac{\text{ N(Required outcome)}}{\text{ N(Total outcome)}}[/tex]
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
[tex]\begin{gathered} \text{ P(select one containing \$491 check) =}\frac{3}{6}=\frac{1}{2} \\ \text{ P(select one containing \$1003 check) =}\frac{3}{6}=\frac{1}{2} \end{gathered}[/tex]
Then, the expected value is given by
[tex]\begin{gathered} \text{ P(Expected Value) =(}\frac{1}{2}\times491)\text{ +(}\frac{1}{2}\times1003) \\ =245.5+501.5 \\ =747.0 \end{gathered}[/tex]