I am assuming that "voice" is boys. So there were 20 girls and 10 boys. The number of girls decreased by 30%, so the amount of girls now is 70% of the old amount, so there are 7/10 * 20 girls now = 14 girls. The number of boys increased by 40%, so the number of boys now is 14/10 the original number of boys, so there are 14 boys now. 14+14=28 members on the swimming team
