## To formulate a function for Plan A, we use the equation: EarningsA = $600 + 0.04x, where x represents the gross sales for the month.

Now, to formulate a function for Plan B, we use the equation: EarningsB = $700 + 0.06(x - $10,000). Here, we subtract $10,000 from the gross sales (x) as it represents the amount of sales that exceeds $10,000.

To determine for what gross sales values Plan B is preferable, we need to compare the earnings under both plans. We set EarningsA = EarningsB and solve for x.

$600 + 0.04x = $700 + 0.06(x - $10,000)

Simplify the equation:

$600 + 0.04x = $700 + 0.06x - $600

Combine like terms:

0.02x = $100

Divide both sides by 0.02 to isolate x:

x = $100 / 0.02

x = $5,000

Therefore, Plan B is preferable for gross sales values greater than $5,000.