__Example 3__Given that ${\mathrm{log}}_{7}4=0.712$ and ${\mathrm{log}}_{7}5=0.827$ , evaluate the following.

(a) ${\mathrm{log}}_{7}20$

(b) ${\mathrm{log}}_{7}1\frac{1}{4}$

(c) ${\mathrm{log}}_{7}0.8$

(d) ${\mathrm{log}}_{7}28$

(e) ${\mathrm{log}}_{7}140$

(f) ${\mathrm{log}}_{7}100$

(g) ${\mathrm{log}}_{7}0.25$

(h) ${\mathrm{log}}_{7}\frac{35}{64}$