3) Swapping two numbers without using a temporary variable  

void main()

{
int a,b;
printf("enter any 2 nums to swap");
scanf("%d%d",&a,&b);
printf("\n nums before swapping are a=%d and b=%d",a,b);

a=a+b; //statement 1

b=a-b;  //statement 2

a=a-b;  //statement 3 

printf("\n nums after swapping are a=%d and b=%d\n",a,b);

}

Lets Take an Example and try to solve see how it really works 

lets say a=2,b=3;

 statement 1

a=a+b  now lets substitute our values and try to evaluate the expression

a=2+3

So at the end of  the statement 1 a's value is 5

statement 2

b=a-b;

b=5-3

b=2

So at the end of the statement 2 b's value is 2

statement 3 

a=a-b

a= 5-2 ( remember a's value is still 5 from our first statement) 

So at the end of the statement a's value is 3 

Mission Accomplished now a=3 and b=2

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