With impedance held fixed, source voltage is the numerator that drives current, so doubling voltage doubles the current magnitude. The 5 ohm impedance gives 1, 2, and 3 A for 5, 10, and 15 V exactly for the ideal same-frequency phasor model; real circuits add impedance tolerance, frequency dependence, parasitics, source limits, heating, loading, bandwidth limits, and voltage/current measurement uncertainty.
A fixed impedance triangle lets source voltage isolate the current numerator. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Five volts through five ohms gives one ampere
The checked impedance stays 5 ohms. Source voltage 5 volts gives current 1 ampere.
I=55​=1
Ten volts through the same impedance gives two amperes
The same triangle denominator 5 ohms with source voltage 10 volts gives current 2 amperes.
I=510​=2
Fifteen volts gives three amperes
With source voltage 15 volts over the same 5 ohms, current is 3 amperes.