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1) 5^(k+1)>3000
=> 5^k>600 (dividing both sides by 5)
This is insufficient since 5^k can be anything greater than 600.
2) 5^(k-1) = 5^k - 500
Therefore 5^k-5^(k-1) = 500
or 5^k ( 1 - 1/5) = 500
or 5^k = 500 * 5 / 4 = 625 which is <1000
Therefore statement 2 is sufficient. Hence answer B
=> 5^k>600 (dividing both sides by 5)
This is insufficient since 5^k can be anything greater than 600.
2) 5^(k-1) = 5^k - 500
Therefore 5^k-5^(k-1) = 500
or 5^k ( 1 - 1/5) = 500
or 5^k = 500 * 5 / 4 = 625 which is <1000
Therefore statement 2 is sufficient. Hence answer B
