Hi, I hope all of you are well. Recently, I start practicing number theory problems, and during this, I face the difficulty of solving the problems that involved using the concept of GCD And LCM. By reading the problem statement I don't get even an idea that how we could use the LCM or GCD techniques here. Recently in Codeforces Round 860 DIV_2, problem "C" involved such a thing, after reading the solution I understood the solution, but I have no clear idea why GCD or LCM work there, So, I can use them in other problems. I know most of the people say to read the properties, but how to apply them? Please help me to figure out it.