A mixture contains milk and water in the ratio 5 : 1. On adding 5 litres of water, the ratio of milk to water becomes 5 : 2. Find the quantity of milk in the original mixture.

In an examination, the ratio of passes to failures was 4 : 1. Had 30 less appeared and 20 less passed, the ratio of passes to failures would have been 5 : 1. How many students appeared for the examination?

In a school, the boys and girls are in the ratio 9 : 5. If there are 425 girls, what is the total number of students in the school?