This video states that the c.g of most non tapered airplane is 25%-30% of wing chord REAR of the leading edge.The aerodynamic center(a.c) of most wing is 25% rear of the leading edge ....Does this mean the a.c can coincide with the c.g in a tailplane?
For longitudinal stability in a conventional wing-and-tail layout, the CG needs to be ahead of the wing's AC. This causes a nose-down moment that is countered by downward lift at the stabilizer, so that the stabilizer will raise the nose as speed builds up. Thus, if the aircraft is disturbed into a dive, the nose will automatically rise as speed builds; conversely, if disturbed nose up, as speed falls off the nose will tend to drop.
Aircraft with active control (especially computer control), as well as some alternative layouts (such as a classic canard) can alter this conventional stability setup -- for instance, in a canard, the stabilizer at the nose lifts, and the CG will be ahead of the leading edge of the wing in most cases.
Yes, the centre of gravity can coincide with the aerodynamic centre of the main wing, particularly because that point is rather unremarkable. For stability, the centre of gravity needs to be ahead of the neutral point, but that is the overall aerodynamic centre if both main wing and horizontal stabilizer are flying at the same coefficient of lift, which means it is behind that of the main wing itself.
Aft limit of 30% of mean aerodynamic chord seems pretty typical, though IIRC the forward limit is usually further forward than 25%.
The aerodynamic center(a.c) of most wing is 25% rear of the leading edge
Yes, but keep in mind that this is only a typical value. In particular supercritical airfoils tend to have it quite a bit further aft. But you rarely see those on straight-wing aircraft; usually on swept-wing ones.