Let x represent the amount of all her sales for the week. We were told that each week she earns 300 dollar plus a 4% Commission of all her sales.
Recall, percentage is expressed in terms of 100. Thus, 4% of her sales would be
4/100 * x = 0.04x
If she earned 327 dollars this week, it means that
0.04x + 300 = 327
0.04x = 327 - 300
0.04x = 27
x = 27/0.04
x = 675
Mia's art sale this week was $675