To find R3, we can use Ohm's Law which states:

V = I * R

Where:
V = Voltage (U) = 60V
I = Current = 1.5A

First, we need to find the total resistance in the circuit. We can do this by adding the resistances in series:

Rtotal = R1 + R2 + R3

Given:
R1 = 15Ω
R2 = 10Ω

Rtotal = 15Ω + 10Ω + R3
Rtotal = 25Ω + R3

Now, we can use Ohm's Law to find the total resistance:

V = I Rtotal
60V = 1.5A (25Ω + R3)

60V = 37.5Ω + 1.5A R3
60V - 37.5Ω = 1.5A R3
22.5V = 1.5A * R3

Now we can solve for R3:

R3 = 22.5V / 1.5A
R3 = 15Ω

Therefore, the value of R3 is 15Ω.