Physics question on amps and ohms?

Question

In an electric circuit, voltage is equal to the product of current and resistance. A radio hobbyist measures a current of 1.2 amps when a 20-ohm resistor is connected in a circuit with a battery of unknown voltage. When the first resistor is replaced with an unlabeled resistor, the current in the circuit is 2 amps. What is the value of the unlabeled resistor?

12 ohms is right, but I got 33.3 ohms.

Answer 1

I got to 12 by looking at the ratio of change in the amps.  You went from 1.2 to 2, which is a change of 0.6.  

Ohms went from 20 to ?.  Multiply 20 by 0.6 and you get 12.

Answer 2

Using Ohm's Law,

V = IR

where

V = voltage

I = current

R = resistance

First step,

V = 1.2 * 20 = 24 volts

This is the circuit's voltage and with a current of 2 amp with an unknown resistor,

24 = 2R

and solving for R,

R = 24/2

R = 12 ohms

Hope this helps.