Question

p:q= 7:8 and q:r=12:7 p:q=?

Answer 1

p:q is already given so you must be asking for p:r
$\frac{p}{q} = \frac{7}{8} \\ \frac{r}{q} = \frac{ 7 }{12} \\ \frac{ \frac{p}{q} }{ \frac{r}{q} } = \frac{ \frac{7}{8} }{ \frac{7}{12} } \\ \frac{p}{r} = \frac{12}{8} = \frac{3}{2}$

Answer 2

umm i think your question is a bit wrong... i think you're supposed to find p:r
so if it is p:r then
 if p:q=7:8 and q:r=12:7
therefore, 
p:r = p:q x q:r = 7:8 x 12:7 = (7x12)/(8x7) = 3/2 = 3:2