Answer:
Total possible number of outcomes = C(24,6) [24 choose 6] 
=24!/(6!18!) 
= 134596 
Out of which there is only one winning combination. 
Therefore we conclude: 
P(win 20000)=1/134596 
P(lose 1)=134595/134596 
and hence the expected value is: 
20000*(1/134596)+(-1)*(134595/134596) 
=-114595/134596 
=-0.8514 (rounded to four places after decimal)
Explanation:
Hope this helped!