Bunuel wrote:

White and black blocks are stacked in a vertical column so that no two blocks of the same color are adjacent. If there are 247 blocks in the stack, how many white blocks are there in the stack?

(1) The top block in the stack is white

(2) There are 5 white blocks in the 10 blocks at the bottom of the stack.

We'll use a simpler case and a drawing to help show us the logic.

This is an Alternative approach.

(1)

Say we had a smaller, odd number of blocks, say 5:

Then w-b-w-b-w would give 3 whites, that is, half of 5 rounded up.

Similarly 7 blocks would give w-b-w-b-w-b-w, or 4 whites, again half rounded up.

So we have 247/2 (rounded up) white blocks.

Sufficient!

(2) Well, in this case we can have

w-b-w-b-w-b-w-b-w-b or b-w-b-w-b-w-b-w-b-w.

Both have 5 white blocks in the lower 10.

But, as we saw in (1), if we start from white we have (half of 247 rounded up) whites meaning that if we start from black we have (half of 247 rounded down).

Insufficient!

(A) is our answer.

_________________

David

Senior tutor at examPAL

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