Q1
The Quotient rule is used if the denominator also contains a variable, since it's a constant here it's "ignored" and just left as 15 and 4 respectively.
Q2
u is a constant here and differentiating a constant gives 0
\frac{d}{dv} (v-u) = \frac{d}{dv}v -\frac{d}{dv}u = 1-0