This is an interview question I came across: find K first digits of the decimal representation of 1/N. It looks like we need just calculate 10^K/N to solve the problem. Does it make sense ? It looks like I am missing something because the solution is too easy.

Just implement grade-school long division:

int value = 1;
bool outputDecimalSeparator = false;
int digitsOutput = 1;
while(digitsOutput <= k) {
    if (value == 0) {
        Console.Write(0);
    }
    else {
        if (value < n) {
            Console.Write(0);
            value *= 10;
        }
        else {
            Console.Write(value / n);
            value %= n;
        }
   }
   if (outputDecimalSeparator == false) {
       outputDecimalSeparator = true;
       Console.Write('.');
   }
   digitsOutput++;
}
Console.WriteLine();

The branch on value == 0 is to detect when 1 / n has a terminating representation of less than k digits.

Here, n is the denominator in 1 / n and k is the number of digits to print in the decimal representation of 1 / n.

Note that by changing value *= 10 to value *= b you can print the b-ary representation of 1 / n as well.