Ten per cent means ten per hundred, so take your tape and divide it into 10 boxes or rectangles because ten tens make a hundred. Each box stands for 10. 300 is three times as big as 100, and so if the tape stood for 300 instead of 100, each box would be worth 3 times as much as 10, that is, 30. If you write 30 in each box and add all the boxes together you would get 300, just like you get 100 when you add all the boxes representing 10 together.