12. Tom had ten sparklers of the same size. Each sparkler took 2 minutes to burn down completely. He lit them one at a time, starting each one when the previous one had one tenth of the time left to burn. How long did it take for all ten sparklers to burn down?
A 18 minutes and 20 seconds
B 18 minutes and 12 seconds
C 18 minutes
D 17 minutes
E 16 minutes and 40 seconds
Solution
B

Tom let the first nine sparklers burn for nine-tenths of two minutes before lighting the next sparkler. Since $9 \times \frac{9}{10} \times 2=\frac{162}{10}=16 \frac{2}{10}$, the first nine sparklers burned for 16 minutes and 12 seconds. The last one burned for the full two minutes so the total time was 18 minutes and 12 seconds.