G7.1 The acute angle formed by the hands of a clock at $3: 30$ p.m. is $A^{\circ}$. Find $A$.
G7.2 If $\tan (3 A+15)^{\circ}=\sqrt{B}$, find $B$.
G7.3 If $\log _{10} A B=C \log _{10} 15$, find $C$.
G7.4 The points $(1,3),(4,9)$ and $(2, D)$ are collinear. Find $D$.
At 3:00 p.m., the angle between the arms of the clock $=90^{\circ}$
From 3:00 p.m. to 3:30 p.m., the hour-hand had moved $360^{\circ} \times \frac{1}{12} \times \frac{1}{2}=15^{\circ}$.
The minute hand had moved $180^{\circ}$.
$$
\begin{array}{l}
A=180-90-15=75 \\
\tan (225+15)^{\circ}=\sqrt{B} \\
\Rightarrow B=3 \\
\log _{10}(75 \times 3)=C \log _{10} 15 \\
\log _{10} 225=C \log _{10} 15 \\
\Rightarrow C=2 \\
\frac{D-9}{2-4}=\frac{9-3}{4-1} \\
D-9=-4 \\
\Rightarrow D=5
\end{array}
$$