unleaded regular - $1.89 per gallon

unleaded premium - $1.94 pr gallon
diesel - $2.10 per gallon

Micah nows that their flower shop's delivery van uses 50 gallon of diesel a week. she estimates to the nearest dollor that they spend $600.00 on diesel each month. what has she done wrong? write a mathematical sentence that wil give her a better estimate.