Solve the first problem 'multiples of 3 and 5'.
Solve the second problem 'Even Fibonacci numbers'.
It is not needed to check nil.
'solution' is better file name rather than 'answer'.
Solve the 3rd problem 'Largest prime factor'.
Solve the problem 4.
Define the maximum and minimum number of 0-digit as 0.
Solve the problem 5 'Smallest-multiple'.
Rewrite C code iteratively.