- Initially all customers paid with credit cards
- 50c charge was added to credit card purchases under two dollars.
- All customers used cash for small purchases (presumably under 2 dollars) while continuing to use credit cards for larger purchases.
- Still, average dollar amount of each cash transaction was almost identical to the average dollar amount of each credit card transaction

This is unexpected, right? All customer used cash for < $2 purchases and card for > $2 purchases then you would expect that average dollar amount of cash transactions would be small but average dollar amount of card transactions would be high.
when would the two averages be almost same? When customer pay cash for some very high value purchases to average out the small purchases.

Say average dollar amount of all purchases is $8.
When the purchase amount is less than $2, customers use cash. When it is higher, they use card. But some customers would need to make cash transactions of say $15 too to average out the small transactions so that the two averages are same.

Cash transactions: $1, $1.5, $1.8, $14.2, $14.5, $15
Card transactions: $8, $8, $8, $8, $8, $8, $8, $8

The two will have the same average.

A. The average dollar amount spent by customers who did not have cash available was higher after the charge was instated than it had been beforehand.

We don't know anything about customers who did not have cash available. Did they spend more before or later, we don't know. All we know is that before, all customers used to pay by card (the ones who had cash and the ones who didn't) and later all customers started paying small amounts by cash.

B. After the charge was instated, customers who used cash for purchases totaling two dollars or more spent more, on average, than customers who used credit cards for these purchases.

Correct. Customers who used cash for higher transactions, spent more than those who used cards. As discussed above, there must have been $15 transactions to make up for the small <$2 cash transactions.

C. The average dollar amount spent in each cash transaction at the convenience store was lower after the charge was instated than beforehand.

Before, there were no cash transactions. If there were a few, we don't know their dollar amounts.

D. A significant majority of purchases made at the convenience store during the month after the charge was instated were made using credit cards.

Cannot say. We know that the average dollar amount is same. Whether there were more cash transactions or card transactions, we don't know.

E. Prior to the charge, the average amount spent on credit card transactions was higher than the average amount spent on cash transactions.

Prior to charge, there were no cash transactions. Even if there were a few, we don't know their average dollar amount.

Answer (B)