4 The probability generating function of the discrete random variable \(Y\) is given by
$$\mathrm { G } _ { Y } ( t ) = \frac { a + b t ^ { 3 } } { t }$$
where \(a\) and \(b\) are constants.
- Given that \(\mathrm { E } ( Y ) = - 0.7\), find the values of \(a\) and \(b\).
- Find \(\operatorname { Var } ( Y )\).
- Find the probability that the sum of 10 random observations of \(Y\) is - 7 .