Answer:
If 99.995 A pass thru a shunt and 5 mA pass thru the meter, the meter will have full scale deflection for a current of 100 A.
By Ohm's Law
99.995 Rs = .005 Rm
Rs = .005 / 99.995 * 120
Rs = .006 ohms
Using a shunt resistance of .006 Ω will produce the required result.