أما إذا كانت مدة الأقساط المشتراة
duration of purchased installments

Premiums of Purchased Term

duration of purchased premiums

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Note added at 37 mins (2013-03-29 20:49:57 GMT)
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http://phx.corporate-ir.net/External.File?t=1&item=VHlwZT0yf...

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Note added at 39 mins (2013-03-29 20:52:26 GMT)
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http://im41.gulfup.com/Ica6j.jpg

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Note added at 45 mins (2013-03-29 20:58:12 GMT)
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http://www.almaany.com/home.php?language=arabic&lang_name=En...

installment payment plan for [a] purchase


Explanation:
<<انا اعتقد ان المقصود ب <<أقساط مدة مشتراه >> هو <<المدة لاقساط البضاعة التي تم شراءها في السابق

Installment Plan (also referred to as installment payment plans):
A credit system by which payment for merchandise is made in installments over a fixed period of time.

I believe that مدة refers to the "fixed period of time" mentioned above.
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Note added at 1 day13 hrs (2013-03-31 09:35:06 GMT)
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Example sentence(s):
University of Hawaii Installment Payment Plan - Continuing students may sign up for the Spring 2013 installment payment plan beginning November 5, 2012. New students may sign up AFTER registering for spring classes. -http://hilo.hawaii.edu/uhh/bo/Insta
The Installment Payment Plan (IPP) assists students in meeting the cost of their higher education by offering a convenient payment option. - http://www.enrollment.vcu.edu/accounting/payment_plan.html

Reference: http://dictionary.reference.com/browse/installment plan
Reference: http://dictionary.reference.com/browse/installment plan