Computing divisibility and multiplication
Review


  see if I use the global variables, nested loops and two functions with parameters correctly



loopCount is a magic variable, its meaning beats me. Is it some kind of debugging assistant?
The indentations seem random picks between 4 and 8 spaces. Stick to 4.
The div method is public, the mul private. Why introduce this inconsistency? 
Give full names to your methods for readabiliy divide, multiply.
The methods are void, printing the results to the console. This limits usability to zero. Separate interaction with the console from the method flow. Use return values to yield results. Let main log these results to the console.
As suggested in the comments, you can optimize division by probing the smallest prime divisor to minimize cycles in the loops.
Be a bit more consistent in aligning brackets with method declarations.
For readability and consistency, add a blank line after the imports and remove that blank line after the last method.