Mary invested a certain sum of money in a bank that paid simple interest. The amount grew to $260 at the end of 2 years. She waited for another 4 years and got a final amount of $380. What was the principal amount that she invested at the beginning? Pick:	A) $ 200	B) $ 220	C) $ 230	D) $ 240	E) $ 250

Rationale: "what shall be the rate of interest . ? does that is not required for the calculation ? not really ! keep in mind that the interest earned each year will be the same in simple interest . at the end of 2 years , amount = $ 260 at the end of 6 years , amount = $ 380 this means she earned an interest of $ 120 in 4 years . or $ 30 in each year . we know that the interest earned each year will be the same . therefore she must have earned $ 60 in 2 years . hence principal amount = $ 260 - $ 60 = $ 200 option a"
Answer: A