The random variable X has the following probability distribution where a and b are some constants : | X | 1 | 2 | 3 | 4 | 5 | | :--- | :--- | :--- | :--- | :--- | :--- | | P(X) | 0.2 | a | a | 0.2 | b | If the mean E(X) = 3, then find values of a and b and hence determine P(X $\geq$ 3).