A company produces two types of widgets: Type A and Type B. Type A widgets weigh 5 pounds each, and Type B widgets weigh 8 pounds each. A shipment must contain at least 15 widgets, and the total weight of the shipment cannot exceed 108 pounds. If $a$ is the number of Type A widgets and $b$ is the number of Type B widgets, what is the minimum number of Type A widgets that can be in a shipment?