There were 3 girls. If the ratio is 10;1, then you’d make it 30;_. To get 30 from ten, you do times 3, and so you do that to the one as well, and 1x3=3
They can prevent the people from using all of a river for transportation but they do provide great ways to wash stuff and provide food!
the ocean become an important area of study around 200 years ago due to fact that it was the time when scientists focused more on the oceanic biosphere. hope this help