$\begin{array}{l}
a)\,\,72,9 \times 99 + 72 + 0,9\\
= 72,9 \times 99 + \left( {72 + 0,9} \right)\\
= 72,9 \times 99 + 72,9\\
= 72,9 \times \left( {99 + 1} \right)\\
= 72,9 \times 100\\
= 7290\\
b)\,\,35:0,2 + 35:0,25 + 35:0,125 - 35 \times 7\\
= 35 \times 5 + 35 \times 4 + 35 \times 8 - 35 \times 7\\
= 35 \times \left( {5 + 4 + 8 - 7} \right)\\
= 35 \times 10\\
= 350\\
c)\,\,1,5 + 2 + 2,5 + 3 + ... + 50,5 + 51 + 51,5\\
= \left( {1,5 + 51,5} \right) + \left( {2 + 51} \right) + \left( {2,5 + 50,5} \right) + ... + \left( {26 + 27} \right) + 26,5\\
\left( {co\,\,50\,\,cap\,so} \right)\\
= 53 + 53 + 53 + ... + 53 + 26,5\,\,\left( {co\,\,50\,\,so\,\,53} \right)\\
= 53 \times 50 + 26,5\\
= 2650 + 26,5\\
= 2676,5
\end{array}$
