AQA A-level Biology inheritance

The first generation of a population of fruit flies had 50 females.
Calculate how many female fruit flies would be produced from this
population in the fifth generation.
You can assume:
• each female produces 400 offspring each generation
• half the offspring produced each generation are female
• there is no immigration or emigration
• no flies die before reproducing.
Show your working
(I am struggling with this question so if someone could answer it with step-by-step working out it would be appreciated, thank you )

50 females and each is assumed to have around 400 offspring with half the offspring being female.
1/2 of 400 is 200 so just keep multiplying my 200 until u get to fifth gen.
2nd gen: 50 x 200 = 10,000
3rd gen: 10,000 x 200 = 2,000,000
4th gen: 2,000,000 x 200 = 400,000,000
fifth gen: 400,000,000 x 200 = 80,000,000,000

80 billion female fruit flies.