Create and solve an appropriate equation: Maria's earnings = Liam's earnings
0.20x = $625+0.10x.
Find x.
0.10x = $625; thus, x = $6250. At this level of food sales, these two people earn the same amount.
For me personally, the easiest way to do this is by isolating the x² term, and finding the square root of both sides. The hardest way (well actually, the longest way) would be to use the quadratic formula. It just complicates things unnecessarily.