easy maths ( quick )

This is actually a physics question considering moment.
Moment is the size of turning effect produces by a force.
Moment is defined as the product of force and the perpendicular distance between the force and the pivot(centre of rotation).
To balance the beam, the beam should not turn anymore
Let the required distance be x
clockwise moment = anticlockwise moment
mgx = Mgd (where m is the smaller mass, M is the larger mass, g is the acceleration
due to gravity, d is the distance between the balance point and mass M)
(100/1000)*(10)*x = (300/1000)*(10)*(10/100)
x = 0.3 m = 30 cm

This is a physics matter called moment. It is calculated as follow:
anticlockwise moment = 300X10 = 3000
clockwise moment = 100 X L = 3000 = anticlockwise moment
L = 30 cm