This isn't hard

$10! = 1 \times 2 \times 3 \times 4 \times 5 \times 6 \times 7 \times 8 \times 9 \times 10$

$=3628800$

10!=3628800

this C program computes factorial by recursion method . double factorial(unsigned int n) { double fact=1.0;

if( n > 1 )
    for(unsigned int k=2; k<=n; k++)
        fact = fact*k;

return f;

}

6! = 720 , 7 8 = 56, 6! 56 = 40320, 9*10 = 90, 40320 * 90 =3628800.