Astep up transformer used in an automobile has a potential difference across the primary of 1800 v and a potential difference across the secondary of 12 v. what is the ratio of the number of turns of wire on the primary to the number of turns on the secondary?

Please don't use a car's ignition coil as an example. Let's just say it's an ordinary transformer. If you connect 1800 volts AC to one side of the transformer and you get 12 volts out of the other side, then the turns of wire are in the same ratio as the voltages ... 1800/12 = 150.
A car coil doesn't work like an ordinary transformer. In a car, you put 12-volt pulses into one side, and you get voltage out of the other side that's high enough to fire spark plugs and ignite gasoline.
So you've actually got the primary and secondary windings labeled in reverse in the question, and you're actually using it as a step-DOWN transformer.