An object `A` is dropped from the top of a `30m` high building and at the same moment another object `B` is projected vertically upward with an initial speed of `15m//s` from the base of the building. Mass of the object `A` is `2kg` while mass of the object `B` is `4kg`. Find the maximum height above the ground level attained by the centre of mass of `A` and `B` system (take `g=10m//s^2`)