Joan took out a mortgage from her local bank. Each monthly mortgage payment she makes must be triple the amount of the previous month's payment. If her first payment is $100, and the total amount she must pay back is $328,000, how many months will it take Joan to pay back her mortgage?
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Joan took out a mortgage from her local bank. Each monthly
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This is a geometrical progression problem which requires the formula for the sum of the terms of a geometrical progression. The sum of n consecutive terms of a geometric progression is given by:varun289 wrote:Joan took out a mortgage from her local bank. Each monthly mortgage payment she makes must be triple the amount of the previous month's payment. If her first payment is $100, and the total amount she must pay back is $328,000, how many months will it take Joan to pay back her mortgage?
- S(n) = a(r� - 1)/(r - 1), where a is the first term, r is the common ratio of the geometric progression and n = number of terms.
We need to determine n.
So, 100*(3� - 1)/(3 - 1) = 328000
--> (3� - 1)/2 = 3280
--> (3� - 1) = 6560
--> 3� = 6561
Now, units digit of any power of 3 is 1 only when the power is a multiple of 4.
So, n must be a multiple of 4.
Only option B works.
The correct answer is B.
Anju Agarwal
Quant Expert, Gurome
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Quant Expert, Gurome
Backup Methods : General guide on plugging, estimation etc.
Wavy Curve Method : Solving complex inequalities in a matter of seconds.
§ GMAT with Gurome § Admissions with Gurome § Career Advising with Gurome §