main() {
    double a;
    a=3669.0;
    int b;
    b=a;
    printf("b is %d",b);
  
}

output is :b is 3669

when you write b=a; then its automatically converted in int

see on-line compiler result : 

http://ideone.com/60T5b

This is called Implicit Type Conversion Read more here https://www.geeksforgeeks.org/implicit-type-conversion-in-c-with-examples/

No need to modify the statement average=(double)(num1+num2)/2; to get expected result inside printf use %f instead of %d

• 1st (num1+num2) is performed, result of this is of integral type. lets say 15. Next when you do (double)15/2 result is of floating type which is 7.500000.
• from previous step average = (double)7.500000 average holds 7.500000 but since you printed in %d you are getting 0 as its undefined behavior. instead use %f

Here is the working one

int main() {
        int num1 = 7, num2 = 8;
        double average;
        average = (double)(num1 + num2)/2;
        printf("average: %f\n", average);
        return 0;
} 

The problem is incorrect use of printf format - use %g/%f instead of %d

BTW - if you are wondering what your code did here is some abridged explanation that may help you in understanding:

printf routine has treated your floating point result of sqrt as integer. Signed, unsigned integers have their underlying bit representations (put simply - it's the way how they are 'encoded' in memory, registers etc). By specifying format to printf you tell it how it should decipher that bit pattern in specific memory area/register (depends on calling conventions etc). For example:

unsigned int myInt = 0xFFFFFFFF;
printf( "as signed=[%i] as unsigned=[%u]\n", myInt, myInt );

gives: "as signed=[-1] as unsigned=[4294967295]"

One bit pattern used but treated as signed first and unsigned later. Same applies to your code. You've told printf to treat bit pattern that was used to 'encode' floating point result of sqrt as integer. See this:

float myFloat = 8.0;
printf( "%08X\n", *((unsigned int*)&myFloat) );

prints: "41000000"

According to single precision floating point encoding format. 8.0 is simply (-1)^0*(1+fraction=0)*2^(exp=130-127)=2*3=8.0 but printed as int looks like just 41000000 (hex of course).