@set2018 to prove if a grammar is unambiguous is undecidable. we can try to see if the given grammar is LL(k) or LR(k) since they are unambiguous. note that unambiguity does not imply the grammar is LL(k) or LR(k). The grammar in the question is not LR(1). so unambiguity should be concluded by careful observation.
Yes. It is unambiguous grammar since for any string no more than $1$ parse tree is possible.
For precedency draw the parse tree and find the depth of operator - and / .
Here "/" having more depth than " - " operator so precedency of" /" is higher than "-".