Many automobiles have an ammeter. if the headlights which draw a current of 8amps, and the radio, which draws a current of 4 amps, are both turned on?

If you're looking for the reading of the ammeter, it should read the sum of the currents, namely 8+4=12 amps, assuming nothing else is drawing a major current.

Note: please post complete and self-contained questions to get the best and fastest responses. 

Google it that might work