The positive integers r, s, and t are such that r is divisible by s
and s is divisible by t. Is r even?
(1) st is odd.
(2) rt is even.
, clearly not sufficient as no info about r
, for example if r=6
then answer is YES but if r=3
then the answer is NO.
. For product of 2 integers to be even either one or both must be even. Can r
not to be even? The only chance would be if t
is even and r
is odd. Let's check if this scenario is possible: if t
is even, so must be s
, as s
is divisible by t
(if an integer is divisible by even it's even too). Now, if s
is even so must be r
by the very same reasoning. So scenario when r
is not even is not possible --> r=even
Do you have an idea about the level of this question ?