Answer: 472

Solution:

We need $20 \mid n$ and $\tau(n) = 20.$ Write $n = 2^a 5^b m$ with $\gcd(m, 10) = 1;$ then $a \ge 2,$ $b \ge 1,$ and $(a + 1)(b + 1)\tau(m) = 20$ with $a + 1 \ge 3$ and $b + 1 \ge 2.$ The factor $a + 1$ must be a divisor of $20$ that is at least $3:$ one of $4,$ $5,$ $10,$ $20.$

If $a + 1 = 4,$ then $(b + 1)\tau(m) = 5$ forces $b = 4,$ $m = 1,$ so $n = 2^3 5^4 = 5000,$ too large. If $a + 1 = 10,$ then $b = 1,$ $m = 1,$ and $n = 2^9 \cdot 5 = 2560,$ too large. The case $a + 1 = 20$ is impossible because $b + 1 \ge 2.$ If $a + 1 = 5,$ then $(b + 1)\tau(m) = 4,$ giving either $b = 3,$ $m = 1,$ so $n = 2^4 5^3 = 2000 \lt 2019,$ or $b = 1$ and $\tau(m) = 2,$ so $m = p$ is a prime other than $2$ and $5$ and $n = 80p \lt 2019,$ i.e. $p \le 25:$ $p \in \{3, 7, 11, 13, 17, 19, 23\}.$

Therefore $$\begin{aligned} S &= 2000 \\ &\quad {}+ 80\tiny(3 + 7 + 11 + 13 + 17 + 19 + 23) \\ &= 2000 + 80 \cdot 93 \\ &= 9440, \end{aligned}$$ and $\frac{S}{20} = 472.$