A sequence $\{a_n\}$ is defined by $a_n=\int_0^1 x^3(1-x)^n dx\ (n=1,2,3.\cdots)$

Find the constant number $c$ such that $\sum_{n=1}^{\infty} (n+c)(a_n-a_{n+1})=\frac{1}{3}$
1. **Define the sequence and integral:**
   The sequence $\{a_n\}$ is given by:
   \[
   a_n = \int_0^1 x^3 (1-x)^n \, dx \quad \text{for} \quad n = 1, 2, 3, \ldots
   \]

2. **Calculate \(a_n - a_{n+1}\):**
   \[
   a_n - a_{n+1} = \int_0^1 x^3 (1-x)^n \, dx - \int_0^1 x^3 (1-x)^{n+1} \, dx
   \]
   Combine the integrals:
   \[
   a_n - a_{n+1} = \int_0^1 x^3 (1-x)^n (1 - (1-x)) \, dx = \int_0^1 x^4 (1-x)^n \, dx
   \]

3. **Integration by parts:**
   Let \( u = x^4 \) and \( dv = (1-x)^n dx \). Then \( du = 4x^3 dx \) and \( v = -\frac{(1-x)^{n+1}}{n+1} \).
   \[
   \int_0^1 x^4 (1-x)^n \, dx = \left[ -\frac{x^4 (1-x)^{n+1}}{n+1} \right]_0^1 + \int_0^1 \frac{4x^3 (1-x)^{n+1}}{n+1} \, dx
   \]
   The boundary term evaluates to zero, so:
   \[
   \int_0^1 x^4 (1-x)^n \, dx = \frac{4}{n+1} \int_0^1 x^3 (1-x)^{n+1} \, dx = \frac{4}{n+1} a_{n+1}
   \]

4. **Relate \(a_{n+1}\) to \(a_n\):**
   From the above, we have:
   \[
   a_n - a_{n+1} = \frac{4}{n+1} a_{n+1}
   \]
   Solving for \(a_{n+1}\):
   \[
   a_{n+1} = \frac{n+1}{n+5} a_n
   \]

5. **Find the general form of \(a_n\):**
   Using the recurrence relation:
   \[
   a_n = \frac{(n+1)!}{(n+4)!} a_1
   \]
   Given \(a_1 = \frac{1}{20}\), we get:
   \[
   a_n = \frac{6}{(n+1)(n+2)(n+3)(n+4)}
   \]

6. **Substitute \(a_n\) into the summation:**
   \[
   \sum_{n=1}^{\infty} (n+c)(a_n - a_{n+1}) = \frac{1}{3}
   \]
   Using the expression for \(a_n\) and \(a_{n+1}\):
   \[
   a_n - a_{n+1} = \frac{6}{(n+1)(n+2)(n+3)(n+4)} - \frac{6}{(n+2)(n+3)(n+4)(n+5)}
   \]

7. **Simplify the summation:**
   \[
   (n+c) \left( \frac{6}{(n+1)(n+2)(n+3)(n+4)} - \frac{6}{(n+2)(n+3)(n+4)(n+5)} \right)
   \]
   Decompose into partial fractions and solve the resulting linear system:
   \[
   (c - 1) \sum_{n=1}^{\infty} \frac{24}{(n+1)(n+2)(n+3)(n+4)(n+5)} + \sum_{n=1}^{\infty} \frac{24}{(n+2)(n+3)(n+4)(n+5)} = \frac{1}{3}
   \]

8. **Solve for \(c\):**
   Using standard partial fractions and solving the linear system, we find:
   \[
   c = 5
   \]

The final answer is \( \boxed{ c = 5 } \)