e, $\frac{15^2.16^4-15^3.16^3}{12^2.20^3-20^2.12^3}=$ $\frac{3^2.5^2.2^{16}-3^3.5^3.2^{12}}{2^4.3^2.2^6.5^3-2^4.5^2.2^6.3^3}=$ $\frac{3^2.5^2.2^{12}.(2^4-3.5)}{2^{10}.3^2.5^2.(5-3)}=$ $\frac{2^2}{2}=4$
$\frac{2.3+4.6+14.21}{3.5+6.10+21.35}=$ $\frac{2.3+2^2.2.3+2.3.7^2}{3.5+2^2.3.5+3.5.7^2}=$ $\frac{2.3.(1+2^2+7^2)}{3.5.(1+2^2+7^2)}=$ $\frac{6}{15}=$ $\frac{2}{5}$