Bài giải:

Biến đổi:

$\overline{1a,26}+\overline{4,b4}+\overline{5,7c}=10,26+a+4,04+\overline{0,b}+5,70+\overline{0,0c}$

                                 $=(a+\overline{0,b}+\overline{0,0c})+(10,26+4,04+5,7)$

                                  $=\overline{a,bc}+20$

Vì $\overline{a,bc}+20$> $\overline{a,bc}+19,99$ nên

$\overline{1a,26}+\overline{4,b4}+\overline{5,7c}$>  $\overline{a,bc}+19,99$