Answer
To convert from grade to radians, we should multiply the grade by $\frac{\pi}{200}$
Work Step by Step
Let $x$ be the grade. Let $y$ be the angle in degrees.
$y = (0.9~x)^{\circ}$
To convert from degrees to radians, we multiply the angle in degrees by $\frac{\pi}{180}$. Let $z$ be the angle in radians.
$z = \frac{\pi}{180}~y$
$z = \frac{\pi}{180}~(0.9~x)$
$z = \frac{\pi}{200}~x$
To convert from grade to radians, we should multiply the grade by $\frac{\pi}{200}$
