Oh, wait wind disturbed question.

When Fly flew from Josh to Mike its speed would $v_{f} + v_{w}$ .

And when it flew towards Josh its speed would be $v_{f} - v_{w}$ .

Let $T_{a}$ be sum of all time it flew towards Mike and $T_{b}$ be sum of all time it flew towards Josh.

$T_{a} + T_{b}$ =Total time till they meet = $\frac {D}{v_{m}+v_{j}} =\frac {3}{5} hr$

$S = T_{a} \times( {v_{f} + v_{w}} )+ T_{b} \times ({v_{f} - v_{w}})$

Notice that the fly started from Josh and all ended at Where Josh and mike meet so the displacement of Josh and fly are same.

Displacement of Josh= Displacement of fly = $v_{j} \times (T_{a}+T_{b}) = T_{a} \times( {v_{f} + v_{w}} )-T_{b} \times ({v_{f} - v_{w}})$ = $(T_{a} -T_{b}) \times {v_{f}} + (T_{a} +T_{b}) \times {v_{w}}$

$(v_{j} -v_{w}) \times \frac {(T_{a} +T_{b})}{v_{f} -v_{w}}=T_{a}-T_{b}$

$\frac {3}{10} hr =T_{a}-T_{b}$

Getting $T_{a} = \frac {9}{20} hr, T_{b} = \frac {3}{20} hr$

$S = T_{a} \times 40 km/h + T_{b} \times 20 km/h =21 km$