$a)\left(4^6\cdot {0,25}^6\right)\cdot \left({10}^6:5^6\right)={\left(4\cdot 0,25\right)}^6\cdot {\left(10:5\right)}^6=1^6\cdot 2^6=1\cdot 2^6=64$

$b)\left(4^5\cdot {7,5}^5\right):\left({27}^5:9^5\right)={\left(4\cdot 7,5\right)}^5:{\left(27:9\right)}^5={30}^5:3^5={\left(30:3\right)}^5={10}^5=100000$