When a wire is connected to 9 volt battery, the current is 0.020 ampere. What is the resistance of the wire?

In order to get the answer to this you must know the
equation that gives you the relationship between voltage, current, and resistance.  That
equation, which is derived from Ohm's law is

R =
v/r

In this equation, I is the current (measured in
amperes), v is the voltage, and r is the resistance.  Now that we know that, we can use
the information that you have provided to solve this
algebraically.

R = 9/.02

When
you do the math here, you get 450

Resistance is measured in
ohms so your resistance here is 450 ohms.

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