Ohms law I = V/R, V = IR

So if I have 10 volts and want to run something at 5 volts, I cut my resistance in half?

because if i have 2 amps and 5 ohms, I have 10 volts? and 2 amps = 10 volts / 2 ohms

But then I noticed when doing the calculations, in a series circuit, if I have 3 LEDs or Motors,

I might start with 10 volts, then subtract chunks of voltage at each LED/Motor, so that the

second motor only has say, 7.5 volts left, and then the third motor might only have 5 volts left.

But with a resistor, how do I subtract 5 volts directly, so that my 10 volt power supply only gives 5 volts to my motor or LED?

If i know my input voltage, and my resistor's ohms, such as 330 ohms, how do I get to 5 volts on the other side?

With 10 volts, its saying a 330 resistor should have .03 amps, but that doesn't tell me how to change my voltage.

If i have a 330 resistor in place and its reading 10 volts, then if...

Now I'm confused. How do I get subtraction when its all multiplication and division? And why does it look like V = IR is saying by cutting my resistor from 330 to 165, I go from 10 volts to 5? Now I'm even more confused.