Why Is this the right answer? How do I calculate this?

A researcher wants to select a sample of 100 subjects from the below population using Inversely Proportional Stratified Sampling. Group Population Size Heavy Drinker 100 Light Drinker 150 Teetotalers 350 What should be sample size for each group? (round to the nearest number). Selected Answer: Answers: Heavy Drinker 17, Light Drinker 25, Teetotalers - 58 Heavy Drinker 48, Light Drinker 32, Teetotalers - 20 Heavy Drinker 55, Light Drinker 30, Teetotalers - 15 Heavy Drinker 51, Light Drinker 34, Teetotalers 15 Heavy Drinker - 17, Light Drinker 25, Teetotalers - 58