ANSWER

a-- 7 bulbs.

b-- 8 bulbs (every bulb).

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A) Let’s name the bulb’s positions by  1,  2,  3,  ...,  13,  14. Let’s begin in any one and let’s jump every 4 positions. In the figure above, left, the start is position 14. We can see that the blue colour sets in 7 different positions. Had we begin in a different possition, the result would have been the same: 7 .

B) Let`s name the positions by 1,  2,  3,  ... 8. Let’s begin in any one of them and let’s jump every 3 positions. In the figure above, right, the start is position 1. We can see that the yellow colour sets in every position. Had we begin in a different position, the result would have been the same: every position.