Question 1209197
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First coupon reduces the buying price from $400 to  (1-0.1)*400 = 0.9*400 = 360 dollars.


Second coupon reduces the buying price from $360 to  (1-0.15)*360 = 0.85*360 = 306 dollars.


The total discount rate of the two coupons is  {{{(400-306)/400}}} = 0.235.


Notice the remarkable fact that this value of 0.235 coincides with

    1 - (1-0.1)*(1-0.75) = 1 - 0.9*0.85 = 0.235.


It explain you an idea on HOW TO calculate an effective discount rate for a chain of discounts.
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Solved.