kobel51 wrote:Is rst=1 ?
1) rs=1
2) st=1

Target question: Is rst=1?

Statement 1: rs = 1
No information about t
INSUFFICIENT

Statement 2: st = 1
No information about r
INSUFFICIENT

Statements 1 and 2 combined
So, rs = 1 and st = 1
This means that rs/st = 1/1
Simplify to get r/t = 1
This tells us that r = t, so this is all that we can conclude one we combine the statements.

If r = t, there are still several values of r, s and t that satisfy the both statements. Here are two:
Case a: r = 1, s = 1 and t = 1, in which case rst = 1
Case b: r = 0.5, s = 2 and t = 0.5, in which case rst = 0.5
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer = E

Cheers,
Brent