Magnetic field and magnetic force

[delsoo]

Homework Statement

PHYSICS MAGNETIC FIELD
two long parallel straight wire are separated by a distance r in vacuum. an electric current flow through each wire in opposite direction, the magnetic flux density at the centre between 2 wires is (2)(ε0)/(πr) (ANS FROM BOOK) ... can someone explain why?

my ans would be 0.5x2x(ε0)/(πr)... because the resultant between 2 wires is repulsive force...

Homework Equations

The Attempt at a Solution

 

[Simon Bridge]

The force between the wires being repulsive or attractive is irrelevant.
Do you know the equation for the flux density at different distances due to a single long wire?