We know that Resistance Formula :
|R = resistance
| ρ = specific resistance
| l = length of the cable
| A = cross section area
We know that resistance is inversely proportional to square of radius of the wire.
So if the radius of cross section is decreased by x then resistance will be increased by x2.
So in the given question initial radius is 9mm and the radius is decreased by 3mm.
Hence the resistance will be increased by 9 times the initial resistance of the single cable.
So the resistance of the new cable will be R= 9*10= 90 ohm