Answer: There are 490 students who are expected to be carrying a backpack.
Step-by-step explanation:
Since we have given that
Number of students who entered the gym = 10
Number of students who carry a back pack = 7
So, part of students who carry a back pack = 
If the number of students were at the assembly = 700
So, Number of students that are expected to be carrying a backpack is given by

Hence, there are 490 students who are expected to be carrying a backpack.