The operative formula is 689((1 - vn)/i) = 24000.
i = 1% per monthly period.
plugging in the numbers gives us vn = .65155.
recall that vn = (1 + i)-n
Taking natural logs of both sides give the equation n*(-.00995) = -.428232 from which we get n = 43.03 periods, or months, which is 3 years and 7 months.

Robert L.
tutor
Are you subtracting .65155 from 1? Also, the ln's of values less than 1 are negative butboth sides cancel out.
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03/22/15
Kenny D.
03/21/15