There were 20 girls and 10 boys on the swim team last Summer. This summer, the number of girls decreased by 30% and the number of boys increased by 40%. How many member are on the swim team this summer?

This is late, but there would be 24 girls and 6 boys, making 30 members of the swim team. You convert the percentages to decimals and multiply what you want the percentage of by the percent. 30% = 0.30. 20 x 0.30 = 6.