Answer:
You should sell above $16666 per week to earn more with straight 6% rate.
Step-by-step explanation:
6% amounts for 6 cents or 0.06 dollars (this is to maintain units) for every dollar you sell.
for 3% is 0.03 dollars per dollar plus the $500.
We represent each situation with an equation
gain_on_6% = 0.06*sales
gain_on_3% = 0.03*sales + 500
Equating these and solving for 'sales' will give us the value in which the gains are equal. Above that 6% will be more profitable.
This is shown in the following plot: