A combined of 55 light bulbs are stored in two boxes; of these, a tota

We are asked for the number not broken in the second box. We know there are 55 bulbs in total and that 7 of them are broken. We know there are 2 broken in the first box, and 5 broken in the second box, (and that the other 48 remaining bulbs are not broken.)

We will be able to compute the number of not broken in the second box if we know the total number in the second box...or if we know the total number in the first box...and, finally, because we know that the total is 55, we will also have sufficiency if we get a special equation relating the total number in the first box to the total number in the second box.


Statement 2: this is a special equation that will allow us to compute the total number in the second box. Once we have that, we would simply subtract the 5 broken, and we'd have the number not broken in the second box. But because this is data sufficiency, instead of actually doing that math, we would just realize (after/during our analysis) that we COULD do it.

So here's the work we wouldn't actually do:

# in 1st box + # in 2nd box =55 (from question)
# in 1st box = # in 2nd box + 9 (statement 2)

subbing in:

(# in 2nd box + 9) + # in 2nd box =55

solving:
# in 2nd box = 23
Therefore, number of not broken in 2nd box is 23-5= 18
Sufficient.

Hence D.

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