Two particles one of mass m and the other 2m are projected horizontally towards each other from the same level of height H above the ground with the velocities 10 m/ s and 5 m/ s respectively at t = 0 . They collide in air and stick to each other. If the total time of motion before it strikes the ground is 8 sec. Find the value of H in meter. (Take g =10 m /s² )