Cutting from two turns to one halves both the linkage and the induced voltage, because linkage scales linearly with turns. One turn gives 1 times 6 equals 6 webers and 6 webers over 3 seconds gives 2 V, exactly half the 4 V from two turns, for the ideal uniform-flux model; real coils add core saturation, leakage flux, winding resistance, frequency dependence, heating, parasitics, and measurement uncertainty.
The one-turn contrast derives a smaller linkage and voltage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
One turn has half the linkage
One turn with the same flux change has linkage 6 webers instead of 12.