A bank pays interest to its customers on the last day of the year. The interest paid to a customer
is calculated as 10% of the average monthly balance maintained by the customer. John is a
customer at the bank. On the last day, when the interest was accumulated into his account, his
bank balance doubled to $4680. What is the average monthly balance maintained by John in his
account during the year?  Pick.
A) 2840  B) 5680  C) 23400  D) 7540  E) 28400
bank balance is doubled with accumulation of interest tp 4680..   this means INTEREST is   4680/2= 
<gadget id="calculator">  4680/2</gadget>
<output>2_340</output>
2340 for entire year..   although since interest is 10% of avg MONthly balance, it becomes 23400..   23400
<result>C</result>
A bank pays interest to its customers on the last day of the year. The interest paid to a customer
is calculated as 10% of the average monthly balance maintained by the customer. John is a
customer at the bank. On the last day, when the interest was accumulated into his account, his
bank balance doubled to $4680. What is the average monthly balance maintained by John in his
account during the year?