#define a 3+2
#define is used to define a constant.
in this program 3+2 is defined for 'a'.
so whenever you use a variable,it'll replace the a with 3+2.
b=a*2; in this expression a is replaced by 3+2 while creating expanded code.
so it'll become like this b=3+2*2;
we know that '*' has high precedence than +.
2*2 will multiplied and it'll added to 3.