After pulling the wire, its length increased by 4 times. What was the resistance of this wire, if it was 20 ohms.

Wire resistance:
R = ρ * l / S, where ρ is the resistivity of the wire material, l is the length of the wire, S is the cross-sectional area of the wire.
Since the volume of the wire has not changed during pulling, and the wire has a cylindrical shape, then:
V1 = l1 * S1, V2 = l2 * S2, where according to the problem statement l2 = 4l1, R = 20 Ohm.
S2 = l1 * S1 / l2 = l1 * S1 / 4l1 = 0.25 S1.
Wire resistance after pulling:
R2 = ρ * l2 / S2 = ρ * 4l1 / 0.25S1 = 16ρ * l1 / S1.
R2 = 16R1 = 16 * 20 = 320 ohms.
Answer: The resistance of the wire is 320 ohms.