Which floor did the flowerpot fall off? It passed by the window of a 5th floor apartment in 0,12 sec. The height of the floors is 3m, the windows 1,2m. (The book says the answer is 7th floor. How can I get it?)

The initial speed is zero because the balls are dropped and not thrown. Therefore, we can solve for time:

let t1 be time to reach the top of window and t2 to reach the bottom of the window. and let d be the distance it has traveled to reach 5th floor window

hence, t1 =sqrt(2d/9.8) and t2= sqrt(2(d+1.2)/9.8)

now we know. t2-t1 = 0.12 = sqrt(2d/9.8) - sqrt(2(d+1.2)/9.8)