find the expected value of each case
1 comes up ---> 1/6 (-10) = -10/6
3 comes up ---> 1/6 (-30) = -30/6
5 comes up ---> 1/6 (-50) = -50/6
2 comes up ---> 1/6 (10) = 10/6
4 comes up ---> 1/6 (20) = 20/6
6 comes up ---> 1/6 (30) = 30/6
sum = 1/6 (-10 -30 -50 +10 +20 +30) = -30/6 = -5