Suppose a bulb passes 3A of current when 120V is put across it, what then is its resistance?
Voltage and current are proportional --- the voltage is multiplied by a factor of 4 so the current is also multiplied by 4: 12A of current will flow.
Transistors are useful because their resistance to current flowing between two of their terminals depends on the current which flows in a third, control terminal.
An electrician holds a (grounded) aluminum ladder with one hand and a live electric light terminal (at 120V) with the other. What will happen to her?
Case 1: Dirty hands. Use V = IR to find
I = V/R = (120V)/(1,000,000) = 0.00012A
--- She does not even notice.
Cases 2: Clean hands.
Find I = V/R = (120V)/(500) = 0.24A
--- She is killed instantly.