Total amount of students = 6
Total amount of girls = 4
So, chance of picking a girl first time is 4/6 or 2/3 chance
But remember! The girl is picked, and we do not want to replace her back into the set. So total amount of students drop to 5 and girls drop to 3
So using that logic, The probability of choosing all girls = 4/6 * 3/5 * 2/4 * 1/3 = 1/15.