caramel3536 wrote:Is rst =1?
(1) rs = 1
(2) st = 1

ans E

(1)
insufficient, because you don't know anything about t

(2)
insufficient, because you don't know anything about r

(together)
if you multiply the two statements together, you get (r)(s^2)(t) = 1, but this isn't the same as rst = 1.
so, try to find contradictory examples:
if r = 1/2, s = 2, t = 1/2, then rst = 1/2 (so doesn't equal 1)
if r = s = t = 1, then rst = 1
insufficient

answer = e

anniev2 wrote:Why can't the equations provided in the statement be plugged into the given:

Is rst=1

(Statement 1) rs=1

Substituting values in equation:
1 x t =1
1t=1
t=1
- if we know that rs = 1 and t = 1 then we know that rst = 1

...
same can be done for the second statement. Why is this unacceptable?

"Is rst = 1?" is the question; it's not a fact that you can use. If you do assume "rst = 1" is true here, it should not be a surprise to discover that rst is equal to 1 -- that's what's called 'begging the question' in logic (assuming the answer to a question is yes, then proving the answer is yes).

When you plug rs = 1 into the question to find "t = 1", all you are doing is rephrasing the question. That is, if we use Statement 1 here, our question becomes: "is t equal to 1?" Since we don't know if that's true, Statement 1 is not sufficient.