Arun and Bilal start together from post X and run to post Y, 600 m away, and straight back, at 6 m/s and 4 m/s. Where do they meet?

At the meeting they have together run two full lengths, $1200$ m, at a combined 10 m/s: 120 s. Bilal has then run $4\times 120=480$ m, still on his way out, so they meet 480 m from X (Arun has run 600 + 120).