INSTRUCTING ARDUINO TO RECALL A PREVIOUSLY GIVEN VALUE

Projects Programming
4 
if (readString.length() >0) {
	Serial.println(readString);
	int n;
	char carray[6];
	readString.toCharArray(carray, sizeof(carray));
	n = atoi(carray);
    n = n * 1000;
    readString="";
    }

n is a local variable. It goes out of scope when this block ends. It can not be referenced outside of this block.

You need to change the scope of the variable. If you want it's value to persist, seriously consider changing its name, too.

Global variables named c or i or n are just recipes for disaster.