Answer: 93
Step-by-step explanation:
Given : The 24 students in Ms.Lee class each collected an average of 18 cans for recycling .
Then , the total cans collected in Ms.Lee class :
Average x Number of students
= 
i.e. Total cans collected in Ms.Lee class =432
Also, the 21 students in Mr Galvez class each collected an average of 25 cans for recycling.
Then , the total cans collected in Mr Galvez class :
Average x Number of students
= 
i.e. Total cans collected in Mr Galvez class =525
Difference = Cans collected in Mr Galvez class - Cans collected in Ms.Lee class
=525-432=93
Hence, Mr. Galvez class collected 93 more cans than Ms.Lee class.