To best determine if the cashier's prediction is likely to be accurate, the question you should ask is: "Are the data sets strongly correlated?"
By knowing the strength of the correlation between the number of customers paying with cash and the number of customers paying with credit, you can assess how reliable the prediction will be. A strong positive correlation would indicate that as the number of customers paying with credit increases, the number of customers paying with cash also tends to increase. In this case, the cashier's prediction would likely be more accurate. On the other hand, if there is a weak or negative correlation, the prediction may not be as reliable.