$3\cdot {10}^7=3\cdot 10000000=300000003\backslash cdot10^7\backslash \; =\backslash \; 3\backslash cdot10\backslash \; 000\backslash \; 000\backslash \; =\backslash \; 30\backslash \; 000\backslash \; 000$

$6.4\cdot {10}^{-4}=6.4\cdot 0.0001=0.000646.4\backslash cdot10^\{-4\}\backslash \; =\backslash \; 6.4\backslash \; \backslash cdot0.0001\backslash \; =\backslash \; 0.00064$

$1.6\cdot {10}^{-6}=1.6\cdot 0.000001=0.00000161.6\backslash cdot10^\{-6\}\backslash \; =\backslash \; 1.6\backslash \; \backslash cdot0.000001\backslash \; =\backslash \; 0.0000016$

$7.4\cdot {10}^9=7.4\cdot 1000000000=74000000007.4\backslash cdot10^9\backslash \; =\backslash \; 7.4\backslash cdot1\backslash \; 000\backslash \; 000\backslash \; 000\backslash \; =\backslash \; 7\backslash \; 400\backslash \; 000\backslash \; 000$