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 $5080. What is the average monthly balance maintained by John in his
account during the year?  Pick:
A) 2840 B) 5680 C) 6840 D) 25400 E) 28400
bank balance is doubled with accumulation of interest tp 5080..   this means INTEREST is   5080/2= 
<gadget id="calculator">  5080/2</gadget>
<output>2_540</output>
2540 for entire year..   although since interest is 10% of avg MONthly balance, it becomes 25400..   25400
<result>D</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 $5080. What is the average monthly balance maintained by John in his
account during the year?