Final answer:
The preferred method of purchase would be to buy the binders in bulk to negotiate a discounted price and save money.
Step-by-step explanation:
In this case, the preferred method of purchase for the training facility would be to buy the binders in bulk. Since they need to purchase 50 binders, buying them individually at $20 each would cost a total of $1000. However, if they buy the binders in bulk, they might be able to negotiate a discounted price. This could result in significant savings.