EXPLANATION
Let's see the facts:
Number of collected signatures=S= 82
Signatures rate: 15 signatures/classroom
C= number of classroom visited
Now, we can assevere that the equation is given by the following relationship:
Total signatures collected = Collected signatures + Rate* Number of classroom visited
Let's call T to the total number of signatures and x to the rate>
The equation is:
T = S + xC