Mary is expected to pay back a total sum of $819200 to her bank as mortgage. If she is expected to make a payment which is double of the previous months amount, how long will it take her to fully repay the loan if her initial payment was $400.  Choose the correct choice from the following.
A) 13	B) 15	C) 12	D) 10	E) 11

Rationale: first payment - - $ 400 total amount to be repaid - - $ 819200 first month payment - - $ 400 second month payment - - $ 400 * 2 = $ 800 third month payment - - $ 800 * 2 = $ 1600 fourth month payment - - $ 1600 * 2 = $ 3200 fifth month payment - - $ 3200 * 2 = $ 6400 sixth month payment - - $ 6400 * 2 = $ 12800 seventh month payment - - $ 25600 * 2 = $ 51200 eight month payment - - $ 51200 * 2 = $ 102400 ninth month payment - - $ 102400 * 2 = $ 204800 tenth month payment - - $ 204800 * 2 = $ 409600 eleventh month payment - - $ 409600 * 2 = $ 819200 answer is e
Answer: E