Let R be a discrete random variable which takes values over {1, 2, 3, 4, 5}

Let R be a discrete random variable which takes values over {1, 2, 3, 4, 5}.
It is known that for each r = 1, 2, . . . , 5, we have P(R = r) = (c × r), for some real value c.

Determine a numerical value for c and E[R].