Advd rotates through an angle of 20π radians in a second. at this speed how many revolutions does the dvd make in 30 seconds?

The correct answer to this question is 300 full revolutions. To solve this, 
(20pi radians / 1 second) x (30 seconds)
= (20pi x 30) radians
(20pi radians / 1 second) x (30 seconds)
= 600pi radians

In 30 seconds, it rotates 600pi radians
2pi radians = 1 rev,  (600pi radians) x (1 rev / 2pi radians) = (600pi / 2pi) rev (600pi radians) x (1 rev / 2pi radians) = 300 rev
In 30 seconds, it completes 300 full revolutions.

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Number nine is f(x)= 40x : ) hoped i ya