Answer: The battery was 19 percent full when the charging began.
Explanation:
Since we have given that
Every minute, the percentage of the battery's capacity increased by = 2.5%
As we know that "Equation of line ":

So, our equation becomes for now,

As we have given that,
After 17 minutes, the battery was 61.5% full.
So, our coordinate becomes (17,61.5)
So, it becomes

Hence, the battery was 19 percent full when the charging began.