A) [40,100] b- [30,100] c- [16,100] d - [11,100]

Hello from MrBillDoesMath!

Choice C   [16,100]

Discussion:

Let "x" be the score needed on the 5th exam to achieve an average score >= 60.  Then

(50 + 90 + 66 + 78 + x)/5 >= 60  =>        (multiply both sides by 5)

(50 + 90 +66 + 78) + x >= 60*5 = 300 =>

(284) + x >= 300 =>

x >= 300 - 284 = 16.

As x is a percent, x <= 100, so the interval is [16, 100], which is Choice C.

Thank you,

MrB

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step-by-step explanation: