$a)\frac{\left(5.1\cdot {10}^6\right)\cdot \left(1.5\cdot {10}^{-5}\right)}{1.7\cdot {10}^{16}}=\frac{5.1\cdot 1.5\cdot {10}^6\cdot {10}^{-5}}{1.7\cdot {10}^{16}}=\frac{5,1}{1,7}\cdot 1,5\cdot \frac{{10}^{6-5}}{{10}^{16}}=$  

$=3\cdot 1,5\cdot \frac{{10}^1}{{10}^{16}}=4,5\cdot {10}^{1-16}=4,5\cdot {10}^{-15}$ $=3\cdot 1,5\cdot \frac{{10}^1}{{10}^{16}}=4,5\cdot {10}^{1-16}=4,5\cdot {10}^{-15}$  

`b) \ \ ((6.5*10^(-12))*(5*10^(-18)))/(2.5*10^(-30))=(5*6.5*10^(-12)*10^(-18))/(2.5*10^(-30))=5/(2,5)*6,5*10^(-30)/10^(-30)=

$=2\cdot 6,5\cdot {10}^{-30-\left(-30\right)}=13\cdot {10}^{-30+30}=1,3\cdot 10\cdot {10}^0=1,3\cdot {10}^1\cdot {10}^0=1,3\cdot {10}^{1+0}=1,3\cdot {10}^1$  

$c)\frac{\left(3.6\cdot {10}^{14}\right)\cdot \left(1.2\cdot {10}^{-12}\right)}{\left(6\cdot {10}^{-6}\right)\cdot \left(0.8\cdot {10}^{12}\right)}=$   $\frac{3,6\cdot 1,2\cdot {10}^{14}\cdot {10}^{-12}}{6\cdot 0,8\cdot {10}^{-6}\cdot {10}^{12}}=$   $\frac{3,6\cdot 1,2\cdot {10}^2}{4,8\cdot {10}^6}=\frac{3,6}{4,8}\cdot 1,2\cdot {10}^{2-6}=$  

$=\frac{36}{48}\cdot 1\frac{2}{10}\cdot {10}^{-4}=\frac{3}{{\sout{4}}^1}\cdot \frac{{\sout{12}}^3}{10}\cdot {10}^{-4}=\frac{9}{10}\cdot {10}^{-4}=0,9\cdot {10}^{1-}\cdot {10}^{-4}=9\cdot {10}^{-5}$