Finding out unknown voltage using mesh analysis
simulate this circuit &ndash; Schematic created using CircuitLab

Applying KVL on Mesh 1 :

80 = 10i1 + 20(i1–i2) + 30 (i1–i3)
80 = 10i1 + 20i1 - 20i2 + 30i1 - 30i3
80 = 60i1 – 20i2 – 30i3... ... (1)


Applying KVL on the Supermesh :

30 = 40i3 + 30(i3–i1) + 20(i2–i1)
30 = 40i3 + 30i3 – 30i1 + 20i2 - 20i1
30 = 70i3 – 50i1 + 20i2 ... ... (2)


Applying KCL at Node1 :

15i1 = i3 – i2
i3 = 15i1 + i2 ... ... (3)


Solving (1), (2) and (3) :

i1 = 0.583 A
i2 = -6.15 A
i3 = 2.6 A


Result : 

v3 = 40 * i3
v3 = 104 V