If 100 students are in a class 40 are boys 30 wear glasses 15 are boys that wear glasses what is the probability that a girl will be chose that does not wear glasses

In a box lenght 1m breadth 60 cm and height 40 cm small rectangular packets of dimension 5 cm into 8 cm into 10 cm are packed. how many such small packets can be packed in the big box?

How do you solve a question like this?

State one method by which you can recognize a perfect square trinomial.