6-faced die; probability of rolling face with number k is c*qk, where c is constant, q = 0.77. Expected value?

[dave92]

I have a die with six faces numbered consecutively from 1 to 6. What is odd about it, is that the probability of rolling the face with number k on it is c*qk, where c is a constant and q = 0.77.

What is the expected value of a roll of the die?

 

[Dr.Peterson]

First, you need to find that constant. What must be true of the probabilities to make this a valid probability distribution?

I don't know why there are two separate constants, c and q.

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