Find the smallest natural number $n$ which has the following properties:

a) Its decimal representation has a 6 as the last digit.

b) If the last digit 6 is erased and placed in front of the remaining digits, the resulting number is four times as large as the original number $n$.
1. Let \( n \) be a natural number with the properties given in the problem. We can express \( n \) in the form:
   \[
   n = 10N + 6
   \]
   where \( N \) is the integer part of \( n \) when the last digit (6) is removed.

2. According to the problem, if the last digit 6 is erased and placed in front of the remaining digits, the resulting number is four times as large as the original number \( n \). This can be written as:
   \[
   4n = 6 \cdot 10^k + N
   \]
   where \( k \) is the number of digits in \( N \).

3. Substitute \( n = 10N + 6 \) into the equation:
   \[
   4(10N + 6) = 6 \cdot 10^k + N
   \]
   Simplify the equation:
   \[
   40N + 24 = 6 \cdot 10^k + N
   \]
   \[
   40N - N = 6 \cdot 10^k - 24
   \]
   \[
   39N = 6 \cdot 10^k - 24
   \]
   \[
   N = \frac{6 \cdot 10^k - 24}{39}
   \]

4. For \( N \) to be an integer, \( 6 \cdot 10^k - 24 \) must be divisible by 39. Simplify the expression:
   \[
   6 \cdot 10^k - 24 \equiv 0 \pmod{39}
   \]
   \[
   6 \cdot 10^k \equiv 24 \pmod{39}
   \]
   \[
   10^k \equiv 4 \pmod{13}
   \]
   (since \( 6 \) and \( 39 \) are coprime, we can divide both sides by 6 modulo 39).

5. We need to find the smallest \( k \) such that \( 10^k \equiv 4 \pmod{13} \). Calculate powers of 10 modulo 13:
   \[
   10^1 \equiv 10 \pmod{13}
   \]
   \[
   10^2 \equiv 100 \equiv 9 \pmod{13}
   \]
   \[
   10^3 \equiv 90 \equiv -1 \pmod{13}
   \]
   \[
   10^4 \equiv -10 \equiv 3 \pmod{13}
   \]
   \[
   10^5 \equiv 30 \equiv 4 \pmod{13}
   \]
   Therefore, \( k = 5 \) is the smallest value that satisfies \( 10^k \equiv 4 \pmod{13} \).

6. Substitute \( k = 5 \) back into the equation for \( N \):
   \[
   N = \frac{6 \cdot 10^5 - 24}{39}
   \]
   \[
   N = \frac{600000 - 24}{39}
   \]
   \[
   N = \frac{599976}{39}
   \]
   \[
   N = 15384
   \]

7. Therefore, the original number \( n \) is:
   \[
   n = 10N + 6 = 10 \cdot 15384 + 6 = 153846
   \]

The final answer is \( \boxed{153846} \).