On a certain date, Pat invested $10,000 at x percent annual interest, compounded annually. If the total value of the investment plus interest at the end of 12 years will be $40,000, in how many years, the total value of the investment plus interest will increase to $160,000?
Select the correct option
A) 15	B) 16	C) 18	D) 20	E) 24
if i were to choose during the test, would go for 18 or 20. probably 18 cuz it wont take too long to get the value doubled....   I found a method: Rule of 72.   Given an x% return, it takes 10,000 to quadralope 12 years.   So according to the rule: 72/x is the no of years 10,000.00 took to double 20,000.00.   Again, 20,000.00 took to double 40,000.00 same (72/x) no of years.   72/x+ 72/x = 12   x = 12% (though rate here is not very much required).   Again, 40,000.00 takes the same (72/x) no of years to double 80,000.00.   72/x = 6 years.   So altogather:   10,000 - 20,000 = 6 years   20,000 - 40,000 = 6 years   40,000 - 80,000 = 6 years   80,000 - 160,000 = 6 years   total 24 years.   Answer 24
<result>E</result>
On a certain date, Pat invested $10,000 at x percent annual interest, compounded annually. If the total value of the investment plus interest at the end of 12 years will be $40,000, in how many years, the total value of the investment plus interest will increase to $160,000?