When a boby is thrown from bottom to top of roof which is 100 m high, at same time another object is thrown downward from roof. At what distance two objects meet each other?

On the ground where they both land?

This will depend on whether the object that is thrown from the roof is given any initial velocity - since you don't mention it I assume it is merely dropped as opposed to being "thrown downward." And It also depends on whether the object thrown up from the ground is given just enough velocity to reach 100 meters altitude. Assuming this is the case - you can set up two equations for the height of each object as a function of time and set them equal. For the object thrown upward you will need to determine the initial velocity required for it to reach a height of 100m. Once you have the two equations set them equal and solve for the time. Then use that time in either of the original equations of motion to get the height at which they meet. Post back with your attempt and we'll check your answer.