Answer:
11.625 Ohm
Explanation:
Let V be the Voltage charge of the loop, as this is constant we know that before the resistor addition the current I is:
V/R1 = 1.9 or V = 1.9R1
After the resistor addition to series R = R1 + 3.1
I = V/R = V/(R1 + 3.1) = 1.5
We can substitute V = 1.9R1
1.9R1 = 1.5R1 + 1.5*3.1
0.4R1 = 4.65
R1 = 4.65/0.4 = 11.625 Ohm