How to determine the voltage Ub2 so that the current through R2 would be equal to 0,150A using Kirchoff's Law? Thank you!

Question

I've drawn the arrows already. I hope they're right. But now I don't know which equations to make?

If it's blurry, here's the given data:
Ub1 = 20V
Ub2 = unknown
R1 = 300 Ohm
R2 = 100 Ohm
R3 = 200 Ohm

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Answer 1 · 2018-02-17T12:59:52

From the directions you have made,we can say, $I_3=I_1 + I_2$

Now,volatge drop across the three wires will be the same,as no battery or resistance is intervening in between them.Let this be $V_a -V_b$

Now,given, $I_2 = 0.150A$

So,voltage drop across $100 ohm$ is $100×0.150=15 V=V_a-V_b$

So,using Kirchoff's law for the upper wire,we get, $V_a-V_b = +20 -I_1×300=15$ or, $I_1=0.017 A$

So, current flowing through $200 ohm$ is $I_1+I_2=(0.017+0.150)=0.167A$

So,using Kirchoff's law,for the lower wire,

$V_a - V_b = -Ub_2 +200×0.167=15$

Or, $Ub_2 = 18.4 V$