Let S be the amount of money the salesperson needs to sell in a week to earn the same amount with the two options. The salesperson earns 40 * $18 = $<<40*18=720>>720 with Option A.
We can set up the following equation to represent the amount of money the salesperson earns with Option B:
0.04S = $720
Solving for S, we find that the salesperson needs to sell $720 / 0.04 = $<<720/0.04=18000>>18,000 to earn the same amount with Option B.