Variables allow you to store data, either from computations in the program or from user input. Variables need to be defined before you can use them, and there are several types to choose from.       Some of the more common variable types include int, char, and float. Each one stores a different type of data. Variables need to be established, or "declared", before they can be used by the program. You declare a variable by entering the data type followed by the variable's name. For example, the following are all valid variable declarations:   float x; char name; int a, b, c, d;    Note that you can declare multiple variables on the same line, as long as they are the same type. Simply separate the variable names with commas. Like many lines in C, each variable declaration line needs to end with a semicolon. Variables must be declared at the beginning of each code block (The parts of your code that are enclosed in {} brackets). If you try to declare a variable later in the block, the program will not function correctly. Now that you know the basics of how variables work, you can write a simple program that will store the user's input. You will be using another function in the program, called scanf. This function searches the input that is provided for specific values.   #include <stdio.h>  int main() {   int x;    printf( "Enter a number: " );   scanf( "%d", &x );   printf( "You entered %d", x );   getchar();   return 0; }    The "%d" string tells scanf to look for integers in the user input. The & before the variable x tells scanf where to find the variable in order to change it, and stores the integer in the variable. The final printf command reads back the input integer to the user. You can use mathematical expressions to manipulate the data that you have stored in your variables. The most important distinction to remember for mathematical expressions is that a single = sets the value of the variable, while == compares the values on either side to see if they are equal.   x = 3 * 4; /* sets "x" to 3 * 4, or 12...

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