The fourth proportional to 4, a, and 16a is 81. What is the value of a?
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Correct answer: C
Explanation
The correct answer is option 3 i.e. 9/2.
The fourth proportional to 4, a, and 16a is 81.
According to the question;
4 : a = 16a : 81
4/a = 16a/81
⇒ 4 × 81 = a × 16a
⇒ 324 = 16a2
⇒ a2 = 324/16
⇒ a2 = 20.25
⇒ a = √20.25
⇒ a = 4.5 = 9/2