Number of girls = 20 and number of boys = 16
Total students = 20 +16 = 36
The number of ways of selecting 2 students from 36 = 36C2
Number of ways selecting two boys from 16 boys = 16C2
Therefore the required probability = 16C2 / 36C2
= (16x15)/ (36x35)
= (4x3)/(9x7)
= 4/21