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.