Answer
The amplitude of the induced $emf$ is $18.0~V$
Work Step by Step
We can write an expression for the induced $emf$:
$emf = \omega~NBA~\vert sin~(\omega~t) \vert$
We can write an expression for the amplitude of the induced $emf$ when $\omega = 15.0~rad/s$:
$emf_1 = (15.0~rad/s)~NBA = 27.0~V$
We can find the induced $emf$ when $\omega = 10.0~rad/s$:
$emf_2 = (10.0~rad/s)~NBA$
$emf_2 = \frac{10.0~rad/s}{15.0~rad/s}~emf_1$
$emf_2 = (\frac{2}{3})~(27.0~V)$
$emf_2 = 18.0~V$
The amplitude of the induced $emf$ is $18.0~V$.
