Answer: 10 seconds
Step-by-step explanation:
m here means mean.
Mean or more accurately arithmetic mean, is calculated by adding all the terms and then dividing by the number of terms.
If there are 5 terms being 4, 5, 5, 6 and a fifth one which is x, and the mean is 6. The relevant expression is:
(4 + 5 + 5 + 6 + x) / 5 = 6
Solving it:
4 + 5 + 5 + 6 + x = 6 * 5
20 + x = 30
x = 30 - 20
x = 10 seconds