Mehul 's activity
One day, Mehul wrote the integers 1, 2, 3, ... ,19, 20 on a blackboard. Then he erases any arbitrary numbers \(a,b\) and replaces them with a new number of value \(a+b-1\). And he repeats it for another 18 times.
Show that there is only one number remain. What is the value of that number?