Giải thích các bước giải:
$20^{21}+20^{22}+20^{23}+..+20^{27}+20^{28}\\
=(20^{21}+20^{22})+(20^{23}+20^{24})+..+(20^{27}+20^{28})\\
=20^{21}(1+20)+20^{23}(1+20)+..+20^{27}(1+20)\\
=21(20^{21}+20^{23}+...+20^{27})\\
Vì\quad 21 \quad \vdots \quad7 \\
\rightarrow 21(20^{21}+20^{23}+...+20^{27})\quad \vdots \quad 7\\
\rightarrow 20^{21}+20^{22}+20^{23}+..+20^{27}+20^{28}\quad \vdots \quad 7$