A wire of uniform crosssection has a resistance of 0.025 . What would be the resistance of a wire of the same material which was twice as long and with twice the diameter?
4 Answers

First of all as a (presumed) physics student, U realize that measured quantities have UNITS and these units should always accompany the numerical in order to identify what the measurement scale is (i.e. units).
So that reported resistance would be: 0.025 Ω {ohms}
Solution to question:
Taking the length factor, first, doubling the length would double the resistance because the wire’s resistance would simply double. A wire’s resistance is directly proportional to its length.
Doubling the diameter would quadruple the wire’s crosssectional area, which would (like water flowing thru a larger diameter pipe) lower its resistance to 1/4 of its former value. Crosssectional area is the critical factor.
The combinational effect of length & diameter changes would be the product of their individual change factors:
2 x (1/4) = 2/4 = 0.5 <= combined factor effect
Therefore the resistance of the longer and thicker wire = 0.025(0.5) = 0.0125 Ω ANS

The answer is 0.0125 (ohms I guess), here is how: Resistance of wire = (resistivity)*(length)/(cross section area), and cross section area = pi*r² = pi*(dia/2)² = (pi/4)*(diameter)².
So if the new length is 2 times, that multiplies resistance x 2, but then dividing by (2*dia)² results in dividing by 4, so the net result is the new wire has half resistance of the old wire, so it is 0.0125 Ω

0.025 is not a resistance, it is just a number. Do you mean 0.025 ohms?
Twice the length –> twice the resistance.
Twice the diameter –> four times the crosssection area –> 1/4 the resistance.
2 * (1/4) = 1/2

R2 = R1*2/2^2= R1/2