boys vs girls|most imp questions

The average score of 84 students (boys and girls) in a test is 57 . The ratio of the number of boys to that of girls is 10 : 11 . The average score of boys is 20% less than that of girls. What is the average score of girls in the test?

Six boys and six girls sit in a row randomly. Find the probability that (i) the six girls sit together,(ii) the boys and girls sit alternately.

The number of ways in which 15 boys and 2 girls can sit in a row such that between the girls at the most 2 boys sit is

The ratio of the boys to girls in a class is 2:3 . If 20% of boys leave this class,then the new ratio of boys to girls is