Consider a population that consists of 600 lives aged 50 and 520 livesaged 60.

Consider a population that consists of 600 lives aged 50 and 520 livesaged 60.
probability: one aged 50 lives to 80 p=0.43732

probability: one aged 60 lives to 80 p=0.47806

and all lives have independent future lifetime random variables.
Calculate the standard deviation of the total number of survivors to age80.

The answer is around 16.7 and I need steps.