In the figure shown, the two projectiles are fired simultaneously. The minimum distance between them during their flight is

Explanation

Talking origin at a and x axis along AB
Velocity of A w.r.t. B
`=20sqrt(3)(cos 60 hat(i)+sin 60 hat(j))-20(cos 150hat(i)+sin 150hat(j))`
`=20sqrt(3)(1/2 hat(i)+sqrt(3)/2hat(j))-20(-sqrt(3)/2hat(i)+1/2 hat(j))=20sqrt(3)hat(i)+20hat(j)`
`tan theta=20/(20sqrt(3))=1/sqrt(3)rArr theta =30^(@)`
so `d_("min")/20=sin theta=sin 30^(@)=1/2rArr d_("min")=10 m`