An iron wire and a copper wire of the same length have the same potential difference applied to them. What must be the ratio of their radii if the currents in the two wires are to be the same? (Ratio of the radius of iron wire to that of copper wire). For the resistivities of iron and copper use: rhoiron = 1.0 times 10-7 capital omega m; rhocopper = 1.7 times 10-8 capital omega m;

Working alone at its constant rate, pump x pumped out ¼ of the water in a tank in 2 hours. then pumps y and z started working and the three pumps, working simultaneously at their respective constant rates, pumped out the rest of the water in 3 hours. if pump y, working alone at its constant rate, would have taken 18 hours to pump out the rest of the water, how many hours would it have taken pump z, working alone at its constant rate, to pump out all of the water that was pumped out of the tank? a) 6b) 12c) 15d) 18e) 24