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(i) $15^8\times 15^{12}÷15^{20}=15^{8+12-20}=15^0=1$

(ii) $25^3\times 25^7÷25^{10}=25^{3+7-10}=25^0=1$

(iii) ${\left(\frac{1}{2}\right)}^6\times {\left(\frac{1}{2}\right)}^3÷{\left(\frac{1}{2}\right)}^8={\left(\frac{1}{2}\right)}^{6+3-8}={\left(\frac{1}{2}\right)}^1=\frac{1}{2}$

(iv) ${\left(\frac{3}{4}\right)}^2\times {\left(\frac{3}{4}\right)}^6÷{\left(\frac{3}{4}\right)}^5={\left(\frac{3}{4}\right)}^{2+6-5}={\left(\frac{3}{4}\right)}^3=\frac{27}{64}$