Finding the mass of an object on a balanced ruler with a pivot

[channel1]

urgent help needed to finding the mass

Homework Statement

We have a ruler with a total length L_r and a pivot at its middle. An object of known mass M_1 is placed a distance L_1 from the end of the ruler, as shown. An object of unknown mass M_2 is placed at the end of the ruler.

http://session.masteringphysics.com/problemAsset/1010957/18/MFS_to_6_b.jpg

Find the mass M_2.
Express the mass in terms of M_1, L_1, and L_r.

Homework Equations

M_1*L_1 = M_2*L_2

The Attempt at a Solution

i tried doing ((M1)*0.5*(Lr)) / (L1 - 0.5Lr) but that's wrong and i don't understand why...

 

[channel1]

bump (please help!)

 

[channel1]

rebump

 

[ideasrule]

M_1*L_1 = M_2*L_2

The Attempt at a Solution

i tried doing ((M1)*0.5*(Lr)) / (L1 - 0.5Lr) but that's wrong and i don't understand why...

L_1 is L1-0.5Lr and L_2 is 0.5*Lr