Simple modular arithmetic for you

$\begin{aligned} 80! & \equiv \prod_{n=1}^{80} n \pmod {10^{20}} & \small \blue{\text{Since 80! has 19 trial zeros}} \\ & \equiv 10^{19} \prod_{n\ \nmid\ 5}^{80} n \pmod {10^{20}} \\ & \equiv \left( 2^{78-19}(3\times3 \times 7 \times 9)^8 \bmod 10\right)\times 10^{19} \pmod {10^{20}} \\ & \equiv \left( 2^{59} \times 3^{32} \times 7^8 \bmod 10\right)\times 10^{19} \pmod {10^{20}} \\ & \equiv \left( 2^3(16^{14}) \times (10-1)^{16} \times (50-1)^4 \bmod 10\right)\times 10^{19} \pmod {10^{20}} \\ & \equiv \left( 8(6) \times 1 \times 1 \bmod 10\right)\times 10^{19} \pmod {10^{20}} \\ & \equiv 8 \times 10^{19} \pmod {10^{20}} \end{aligned}$

Therefore $x = 8 \times 10^{19}$ and the sum of digits of $x$ is $\boxed 8$ .