Answer: She will earn $15 excluding the one-time credit during the year.
Explanation:
Since we have given that
Amount Sandra gain = $210
Gain included a one time per year credit
Amount of that one time per year credit = $30
Her equation becomes,

To project how money x her investment account will earn during the year
We need to find the amount that Sandra's account will earn during the year, excluding the one time credit
So, our equation becomes,

Hence, She will earn $15 excluding the one-time credit during the year.