An electromagnet is created using a battery, an insulated copper wire and an iron nail. The wire is wrapped around the nail 30 times and attached to the battery. Which pair of changes will definitely increase the strength of the electromagnet?

Explanation:

If one assume that each turn is like a strand of electromagnet, which can then be added up. Therefore, increase in the number of turns will yield to increase in the magnetic strength. Also if the current increases, then there will be increase in the magnetic field strength.

From Ohm's law

V = IR

I = V/R

That is a direct increase in voltage will lead to increase in current.

Increase the voltage of the battery and increases the number of turns of the coil. Will suit the situation

c. newton's third law of motion

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form