Consider the natural numbers written in the decimal system.
(a) Find the smallest number which decreases five times when its first digit is erased. Which form do all numbers with this property have?
(b) Prove that there is no number that decreases $12$ times when its first digit is erased.
(c) Find the necessary and sufficient condition on $k$ for the existence of a natural number which is divided by $k$ when its first digit is erased.
### Part (a)
1. Let \( N \) be a number such that \( N = x \cdot 10^n + y \), where \( 1 \leq x \leq 9 \) and \( y < 10^n \).
2. The number \( N \) decreases five times when its first digit is erased. This means:
   \[
   x \cdot 10^n + y = 5y
   \]
3. Rearrange the equation to isolate \( y \):
   \[
   x \cdot 10^n = 5y - y \implies x \cdot 10^n = 4y \implies y = \frac{x \cdot 10^n}{4}
   \]
4. For \( y \) to be an integer, \( x \cdot 10^n \) must be divisible by 4. Since \( 10^n \) is always divisible by 4 for \( n \geq 2 \), \( x \) must be even.
5. The smallest even \( x \) is 2. Therefore, the smallest \( N \) is:
   \[
   N = 2 \cdot 10^n + \frac{2 \cdot 10^n}{4} = 2 \cdot 10^n + \frac{10^n}{2} = 2 \cdot 10^n + 0.5 \cdot 10^n = 2.5 \cdot 10^n
   \]
6. The smallest \( n \) for which \( 2.5 \cdot 10^n \) is an integer is \( n = 1 \). Thus, the smallest \( N \) is:
   \[
   N = 2.5 \cdot 10^1 = 25
   \]
7. Therefore, all such numbers \( N \) must be in the form:
   \[
   12\underbrace{00\ldots00}_n, 24\underbrace{00\ldots00}_n, 36\underbrace{00\ldots00}_n, 48\underbrace{00\ldots00}_n
   \]

The final answer is \( \boxed{ k = x \cdot 2^a \cdot 5^b } \), where \( 1 \leq x \leq 9 \) and \( a + b > 0 \)