Every employee at a manufacturing plant is either a manager or a line worker. The plant currently has a ratio of \(4\)

What we see: a Data Sufficiency question that asks for one value: the number of line workers, when managers to line workers is 4 to 9 and every employee is one or the other.

Step 1. Turn the ratio into whole numbers.
What we see: 4 managers for every 9 line workers.
What it means: managers \(= 4k\) and line workers \(= 9k\) for some whole number \(k\). The total is \(13k\), so the number of employees must be a multiple of 13.
What we do and why: the whole question comes down to \(k\). Any statement that fixes \(k\) fixes the answer.

Step 2. Statement 1.
What we see: the plant has between 40 and 60 employees.
What it means: the multiples of 13 near that range are 39, 52 and 65. Only 52 is between 40 and 60. So \(k = 4\) and there are \(9 \times 4 = 36\) line workers.
What we do and why: one value, so statement 1 is sufficient. A range became a single number because people come in whole numbers.

Step 3. Statement 2.
What we see: adding 4 managers would make 20 managers.
What it means: there are 16 managers now. \(4k = 16\), so \(k = 4\) and again 36 line workers.
What we do and why: one value, so statement 2 is sufficient on its own too.

The trap: calling statement 1 not sufficient because "a range can't give one value." It can here, since the total must be a multiple of 13 and only one multiple fits.
Faster way: write the total as \(13k\) the moment you see the ratio. Then statement 1 is a 10 second check of multiples of 13.
Takeaway: A ratio of people or objects means the total is a multiple of the ratio's sum. Test any range against those multiples before you call it not sufficient.

Answer: D