$\begin{array}{l}
a)\,\,\,A = 5 + 8 + 11 + 14 + + ..... + 302\\
Ta\,\,thay\,\,A\,\,la\,\,tong\,\,cua\,\,cac\,\,so\,\,cach\,\,nhau\,\,3\,\,don\,\,vi.\\
So\,\,cac\,\,so\,\,hang\,\,cua\,\,A:\,\,\,\,\left( {302 - 5} \right):3 + 1 = 100.\\
\Rightarrow A = \left( {302 + 5} \right).100:2 = 15350.\\
b)\,\,\,B = 7 + 11 + 15 + 19 + .... + 203\\
Ta\,\,thay\,\,B\,\,la\,\,tong\,\,cua\,\,cac\,\,so\,\,cach\,\,nhau\,\,4\,\,don\,\,vi.\\
So\,\,cac\,\,so\,\,hang\,\,cua\,\,B:\,\,\,\,\left( {203 - 7} \right):4 + 1 = 50.\\
\Rightarrow A = \left( {203 + 7} \right).50:2 = 5250.
\end{array}$
