a, $\frac{2^5 . 15^3 }{6^3 . 10^2}$
= $\frac{2^5 . 3^3 . 5^3}{3^3 . 2^3 . 2^2 . 5^2}$
= $\frac{2^5 . 3^3 . 5^3}{3^3 . 2^5 . 5^2}$
= 5
b, [-3 - $\frac{1}{2}$}] : $\frac{21}{4}$ + $\frac{3}{4}$
= $\frac{7}{2}$ . $\frac{4}{21}$ + $\frac{3}{4}$
= $\frac{7}{2}$ . $\frac{2 . 2}{3.7}$ + $\frac{3}{4}$
= $\frac{2}{3}$ + $\frac{3}{4}$
= $\frac{8}{12}$ + $\frac{9}{12}$
= $\frac{17}{12}$
