a)

$s=12km$

$V=6\frac{km}{h}$

$t=\frac{s}{V}=\frac{12km}{6\frac{km}{h}}=2h$

b) 

$s=100m$

$V=20\frac{km}{h}=20\cdot \frac{1000m}{h}=20000\frac{m}{h}$

$t=\frac{s}{V}=\frac{100m}{20000\frac{m}{h}}=0,005h=0,3\min =18s$

c)

$s=2km$

$V=60\frac{km}{h}$

$t=\frac{s}{V}=\frac{2km}{60\frac{km}{h}}=0,033h=2\min$

d)

$s=2000m=2km$

$V=500\frac{km}{h}$

$t=\frac{s}{V}=\frac{2km}{500\frac{km}{h}}=0,004h=0,24\min =14,4s$