A classroom has 57 students. If there are 19 girls in the class and the number of boys is 'x', find the value of 'x'.
40% of students in a classroom are girls. If there are 90 students in the class, how many of them are boys?
The ratio of number of boys and girls is 4 : 3. If there are 18 girls in a class, find the number of boys in the class and the total number of students in the class.
There are 21 girls and 33 boys in a classroom. A student is randomly selected to become the class leader. If I want to find the probability that the selected student is a girl, the number of favourable outcomes is ___.