C) 18 minutes
This is because each sparkler takes 2 minutes to burn so 10 * 120 = 1200 seconds, then 120 / 0.10 = 12 (which is the 10th of the 2 minutes) now we do 12 * 10 = 120 to get the cut time from lighting a match after a tenth remained. now since we have that we do 1200 - 120 and get 1080 seconds and that is the total time n seconds of the burning matches, now we convert it to minutes by doing 1080 / 60 and get 18.
hope this is helpful!
