How to find the voltage drop in this circuit?
At loop 1: 10v-200I1-Vo=0

At loop 2: 10v-100I2-5v=0

At loop 3: 5v+100I3+vo=0

Now find the value of I2 from the above equation 2 to and put to rest of equations, you will get same number of equations and variable.

I would suggest to go through the Article about Mesh Analysis.