The sequence of flashing would be:
1 -> 4 -> 7 -> 2 -> 5 -> 8 -> 3 -> 6

so D

logical question
draw fig and solve
we see bulb 6 flashes last
IMO D

How can all bulbs flash
The question says that every third bulb will flash. So If it starts from bulb no 1 then the third bulb will be bulb no 3
How can the third bulb be no 4

I hope to help you with this.
In general, every third means that the difference is 3

on a number line, every third number is 3,6,9,..... --> if the initial integer is 0
but if the initial integer is 1, then the sequence is 4,7,10,...

if you assume a sequence of 1,3,6,9,.. ---> it will be an incorrect sequence as the difference between the second and the first is not equal the difference between the third and the second.
if you assume a sequence of 1,3,5,... ---> it will be a sequence of every second number (as the difference is 2)

in other words: if the bulb no. 1 is not the first to flash, then the first one to flash will be bulb no. 3 followed by 6.

Ans: just deduct 3 places from First bulb, because flashing will stop before it reaches the first bulb.
( save time if lot of numbers are present)
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I would like to do it in manual way

Lighting happens like this

1, 4, 7, 2, 5, 8, 3, 6

i.e. Bulb 6 flashes last

Answer: Option D
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"The bulbs are wired so that every third bulb, counting in a clockwise direction, flashes until all bulbs have flashed once. If the bulb numbered 1 flashes first, which numbered bulb will flash last?"

Every third bulb means that the number 1 bulb would be counted as 0... and we would move 3 spaces to number 4 and repeat until none are left. In other words, Last-First + 1 ... would be 4th bulb - 1st bulb + 1 = 3 spaces moved. Repeat until finished.
I drew a diagram and just crossed out the successive bulbs.

Solution

Given
In this question, we are given

• Eight light bulbs numbered 1 through 8 are arranged in a circle, as shown in the diagram.
• The bulbs are wired so that every third bulb, counting in a clockwise direction, flashes until all bulbs have flashed once.
• The bulb numbered 1 flashes first.

To Find
We need to determine

• The number of the bulb that will flash last.

Approach & Working
As every third bulb is flashing, starting from bulb 1, we can determine the next bulbs as follows

• First: Bulb 1
• Second: Bulb 4
• Third: Bulb 7
• Fourth: Bulb 2
• Fifth: Bulb 5
• Sixth: Bulb 8
• Seventh: Bulb 3
• Eighth: Bulb 6

As we can see, all the eight bulbs are flashing at least once, and the last one to flash is bulb 6.

Hence, the correct answer is option D.


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So the bulbs will flash in the following order:

1, 4, 7, 2, 5, 8, 3 ,6

Answer: D
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chetan2u VeritasKarishma Bunuel generis

How is bulb numbered '3' not the 3rd bulb.

1st (Bulb 1), 2nd (Bulb 2), 3rd (Bulb 3).

I have seen MahmoudFawzy's explanation but e.g given a sequence 123456789 and asked to pick every 3rd number, I will pick 3,6 and 9. Will that be wrong ?

Is this something that one is tested on and is expected to be clear (provided that there is a 'standard') ? Shouldn't GMAT not give answer choices which work with both approaches (i.e in this case choice D and E)

Will appreciate some help.

Yes if you are asked to pick every third number in 123456789 will be 3,6,9. But if you are told that you pick 1 and then every third, the numbers would be 1,4,7.
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Every third member means "out of every 3, only 1". It doesn't mean that it must be a multiple of 3.
Which one, depends on which you start with. If I pick 1, then I will skip 2 and 3 and the next third member will be 4.
If I pick 2 first, I will skip 3 and 4 and the next third member will be 5. If I skip 1 and 2 and pick 3 first, then I will skip 4 and 5 and pick 6th.
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