Hi abhasjha,

In this prompt, we're told that a manufacturer had to produce 1,000 tools/day for N days to complete an order on schedule. We're asked for the value of N, but we could rewrite the question (What total number of tools are needed to complete the order?).

Fact 1: The manufacturer produced only 600 tools/day for the FIRST 5 days.

From this, we know that the order had to be greater then 5(600) = 3000 tools. Since there were 5 days of work, the order has to be at least 5000 tools, but it could have been more. There's no way to know what N is.
Fact 1 is INSUFFICIENT

Fact 2: The manufacturer produced 1500 tools/day for the LAST 4 days.

From this, we know that the order had to be greater than 4(1500) = 6000 tools. We run into the same problem as in Fact 1 though, we don't know what the total order was (or could have been).
Fact 2 is INSUFFICIENT

Combined, we know the "beginning" and "end" of the production run, but we don't know about the "middle." There could have been any number of days between the first 5 and the last 4 (and any number of tools could have been produced on those days) but there's no way to determine the actual number of the original order.
Combined, INSUFFICIENT

Final Answer: [spoiler]E/spoiler]

GMAT assassins aren't born, they're made,
Rich