Pricing test question - confidence of conclusion


A chain of car salesrooms splits it's branches into 2 groups for a 3 month period:

1. 100 branches have an average sale price of $16,000
2. 25 branches undergo sales training and then get an average sale price of $16,250.

Assuming I can get every sale price of every individual sale for each group, how should I go about calculating the % confidence that the sales training increased the average sale price? (and that it wasn't just chance)

Thank you,