Mean of Distribution

Three tables listed below show "random variables" and their " probabilities.  However, only one of these is actually a probability distribution.

A) What is  it?

Table 1               Table 2                     Table 3

X   P(X)              X     P(X)                 X   P(X)

5     .3                 5       .1                   5     .5
10   .3                 10     .3                   10    .3
15   .2                 15     .2                   15   -.2
20   .4                 20     .4                  20     .4

B) Using the correct probability distribution, find the probability that x is:

     1. Exactly 15.
     2. No more than 10.
     3. More than 5.

C) Compute the mean, variance, and standard deviation of this distribution