Answer:
11
Step-by-step explanation:
The average price of the 20 shirts is 600/20 = $30 each. So, we know that we can sell 20 shirts for $30 each to reach the goal.
For each $25 shirt we sell, we must also sell a $35 shirt to make the average price of the sale be $30 per shirt. That is any or all of the 10 pairs of $30 shirts can be replaced by a ($25, $35) pair. So, there are 10 additional ways to reach the goal.
The total number of possible combinations of shirt sales is 1 +10 = 11.
