How many Folsom customers prefer to pay with Visa?
Folsom refers to City = 'Folsom'; Visa refers to CreditCardType = 'Visa';
SELECT COUNT(T1.CustomerID) FROM customers AS T1 INNER JOIN `transaction` AS T2 ON T1.CustomerID = T2.CustomerID WHERE T1.City = 'Folsom' AND T2.CreditCardType = 'Visa'