Length of the arc is
, where
is the radius of the circle and
is the central angle.
Relation between linear speed
of an object and angular speed
of the object is
, where
is the radius of the circular motion and
is measured in radians per unit time.
Angular speed
, where
is the angle generated by the object and
is elapsed time.
Radius of the first pulley is
.
Angular speed of first pulley is
rpm.
Arc length of the first pully is
.
Apply derivative on each side with respect to time
.

= constant.
\
Radius of the second pulley is
.
Angular speed of second pulley is
rpm.
Arc length of the first pully is
.
Apply derivative on each side with respect to time
.

= constant.
Since the two pulleys have constant linear speed, equate the velocities.
\
.
.