What is the firing order for an 18-cylinder radial engine if you start with cylinder 1 and add 11 or subtract 7 to stay between 1 and 18?

• A. 1-12-5-16-9-2-13-6-17-10-3-14-7-18-11-4-15-8
• B. 1-13-4-16-7-2-12-5-17-9-3-15-6-18-10-3-11-8
• C. 1-11-3-15-7-2-12-6-17-9-4-14-5-18-13-2-16-8
• D. 1-12-6-15-8-2-13-5-18-10-3-14-7-17-11-4-16-9

Answer: (A)

Firing in this pattern uses modular stepping: start at cylinder 1 and repeatedly add 11, wrapping around when you exceed 18 so you stay within 1 to 18. Because 11 and 18 share no common factor, this step will visit every cylinder exactly once before returning to the start.

Following the jump:

1 -> 12 (1+11)

12 -> 5 (23→wrap)

5 -> 16

16 -> 9

9 -> 2

2 -> 13

13 -> 6

6 -> 17

17 -> 10

10 -> 3

3 -> 14

14 -> 7

7 -> 18

18 -> 11

11 -> 4

4 -> 15

15 -> 8

and then 8 -> 1 would complete the cycle.

So the firing order is 1-12-5-16-9-2-13-6-17-10-3-14-7-18-11-4-15-8. This matches the given correct sequence. The other options don’t follow this +11-with-wrap pattern and thus don’t produce a full 18-cylinder cycle.