The cost of 15 meters of ribbon at the factory is:
15 meters / 3 meters per $7 = 5 times $7 = $35
The cost of 15 meters of ribbon at the store is:
15 meters x $2.50 per meter = $37.50
Therefore, the amount saved by buying 15 meters of ribbon at the factory rather than at the store is:
$37.50 - $35 = $2.50