We know T=2π√lg.
Thus, T is inversely proportional to square
root of g.
For, time period to become T/2, we have geff=4g
beacause 2π√l4g=T/2
Thus, to get
geff=4g, lift should be moving upward with 3g, so that net
acceleration of pendulum should be 3g(downward due to pseudo
force)+g(downward due to gravity)=4g