A salesman gets paid on commission, and his commission is based on how many units he sells during the month. If the salesman sells $1500 worth of units or less, he makes a flat commission of $900 plus 10% of the sale. If he sells more than $1500 worth of units, he makes an additional 6% on anything above $1500. If Mark's months earnings E were $2100 for this month, what was the total value of his sales, s?

To determine the total value of Mark's sales (s), we need to use the given information about his commission structure.

Let's break down the problem into two scenarios:

1. Mark sells $1500 worth of units or less:
In this case, Mark earns a flat commission of $900 plus 10% of the sale.
Since his total earnings (E) are $2100, we can set up the equation:
E = $900 + 10% of the sale
$2100 = $900 + 0.10s

Simplifying the equation:
0.10s = $2100 - $900
0.10s = $1200

Dividing both sides by 0.10:
s = $1200 / 0.10
s = $12,000

Therefore, if Mark sells $1500 worth of units or less, the total value of his sales would be $12,000.

2. Mark sells more than $1500 worth of units:
In this scenario, Mark earns a flat commission of $900 for the first $1500 of sales and an additional 6% on anything above $1500.
Let x represent the amount of sales above $1500, so s = $1500 + x.

The total earnings (E) can be expressed as:
E = $900 (flat commission for first $1500) + 6% of x (additional commission on sales above $1500)

Since E = $2100, we can set up the equation:
$2100 = $900 + 0.06x

Simplifying the equation:
0.06x = $2100 - $900
0.06x = $1200

Dividing both sides by 0.06:
x = $1200 / 0.06
x = $20,000

Therefore, if Mark sells more than $1500 worth of units, the total value of his sales would be $20,000.

In conclusion, to find the total value of Mark's sales (s), we have two possibilities:
- If he sells $1500 or less worth of units, s = $12,000.
- If he sells more than $1500 worth of units, s = $20,000.