1. 3 girls: 9 boys
2. 4 books: 16 pupils
3. 18 notebooks: 6 girls
4. 10 bags : 6 pencils
5. 6 erasers : 2 blackboards

In each of the ratios, the numbers are given in whole numbers, So the ratios are already simplified.

It's important to keep in mind that when simplifying ratios, you should divide both numbers by the same number to keep the proportion consistent.