Answer:
t = 16,1 minutes
Explanation:
For a battery of 6,45V the 90,0% of its voltage is:
6,45V×0,900 = 5,805V
The voltage decreasing of the batteries could be written as:
[tex]V_{t} = V_{i} - 0,040V/min*t[/tex]
Where t is discharge time .
As initial voltage is 6,45V and Vt is 5,805V:
[tex]5,805V = 6,45V - 0,040V/min*t[/tex]
[tex]-0,645V = - 0,040V/min*t[/tex]
t = 16,1 minutes
I hope it helps!