a right angle is 90°, or 1/4 of a circle.
3/5 * 1/4 = 3/20
radian measure is the length of the circumference of this part of the circle.
a full circumference of a circle would be 2*pi*r, also expressed as 2 rad * r
now let's multiply the full thing with the 3/20 from earlier to just get the desired fraction:
6/20 rad * r
= 3/10 rad * r
of course a solution in cm or inch still depends on the radius here, wich could be just plugged in at will