Four equal-value resistors are in a series with a 5v source and a current of 1 mA is measured What is the value of each resistor?

1.25k

Following Ohms Law
Equation:
R = E / I
(R=Resistance, E=Voltage, I=Current)

Known Values:
E = 5v
I = 1mA = 0.001A

R = 5/0.001
R=5000
Total Resistance is 5000ohms, since we know there are four equal resistors simply divide by 4
R/4=5000/4

R=1250ohm=1.25kohm