Solve the Problem $\mathrm { f } ( \mathrm { x } )$

Multiple Choice

Solve the problem.
-In the following formula, $\mathrm { f } ( \mathrm { x } )$ is the minimum number of hours of studying required to attain a test score of $x : \quad f ( x ) = \frac { 0.41 x } { 100.5 - x }$ . How many hours of study are needed to score 91 ?

A) $9.62 \mathrm { hr }$
B) $39.30 \mathrm { hr }$
C) $3.93 \mathrm { hr }$
D) $100.05 \mathrm { hr }$