Mary Robertson's total commission for selling $3,400 worth of tires last week amounted to $480.
Mary Robertson's commission can be calculated by applying the different commission rates to the corresponding sales tiers.
For the first $1,000 in sales, Mary earns 10%, which is $100 (10% of $1,000).
For the next $2,000 in sales, she earns 15%, which is $300 (15% of $2,000).
The remaining $400 in sales (total sales of $3,400 - $1,000 - $2,000) falls into the third tier where Mary earns 20%, which is $80 (20% of $400).
Adding these commissions together, Mary's total commission is $100 + $300 + $80 = $480.
Therefore, Mary Robertson's total commission for selling $3,400 worth of tires last week was $480.
Complete question:
Mary Robertson sells tires at a large car service center. She earns 10% commission on the first $1,000, 15% on the next $2,000, and 20% on sales of more than $3,000. Last week she sold $3,400 worth of tires. What was her total commission?