[tex]\displaystyle\it\\2021\cdot2^{2021}\cdot5^{2020}=2021\cdot2\cdot2^{2020}\cdot5^{2020}=\\4042\cdot10^{2020}=10\underbrace{0000...00}_{2020~de~0}\cdot4042=4042\underbrace{00000...000}_{2020~de~0}~asadar~suma~cifrelor~\\este~4+4+2=\boxed{\it 10}~.[/tex]