$\text{a)}\left(2,5\cdot {10}^{21}\right)\cdot \left(4\cdot {10}^{-3}\right)=\left(2,5\cdot 4\right)\cdot \left({10}^{21}\cdot {10}^{-3}\right)=10\cdot {10}^{18}={10}^{19}=1\cdot {10}^{19}$     

$\text{b)}\left(1,2\cdot {10}^{-25}\right)\cdot \left(0,5\cdot {10}^{27}\right)=\left(1,2\cdot 0,5\right)\cdot \left({10}^{-25}\cdot {10}^{27}\right)=0,6\cdot {10}^2=6\cdot {10}^1$  

$\text{c)}2,1\cdot {10}^{15}+11,9\cdot {10}^{15}=\left(2,1+11,9\right)\cdot {10}^{15}=14\cdot {10}^{15}=1,4\cdot 10\cdot {10}^{15}=1,4\cdot {10}^{16}$  

$\text{d)}17,9\cdot {10}^{-1}+26,5\cdot {10}^{-2}=1,79+0,265=2,055=2,055\cdot {10}^0$  

$\text{e)}52\cdot {10}^{-2}-78\cdot {10}^{-3}=0,52-0,078=0,442=4,42\cdot 0,1=4,42\cdot {10}^{-1}$  

$\text{f)}2,5\cdot {10}^{-3}-35\cdot {10}^{-5}=2,5\cdot {10}^{-3}-0,35\cdot {10}^{-3}=\left(2,5-0,35\right)\cdot {10}^{-3}=2,15\cdot {10}^{-3}$  

$\text{g)}\left(1,3\cdot {10}^{14}\right):\left(2,6\cdot {10}^{10}\right)=\frac{{\sout{1,3}}^1\cdot {10}^{{\sout{14}}^4}}{{\sout{2,6}}^2\cdot {\sout{{10}^{10}}}^1}=\frac{{10}^4}{2}=\frac{1}{2}\cdot {10}^4=0,5\cdot {10}^4=5\cdot {10}^3$  

$\text{h)}\left(2,4\cdot {10}^{-37}\right):\left(0,06\cdot {10}^{-32}\right)=\left(2,4\cdot {10}^{-37}\right):\left(0,6\cdot {10}^{-33}\right)=\frac{{\sout{2,4}}^4\cdot {10}^{{\sout{-37}}^{-4}}}{{\sout{0,6}}^1\cdot {\sout{{10}^{-33}}}^1}=4\cdot {10}^{-4}$