We know that time period of pendulum is:
T = 2 π√lg
As per the above relation,, time period is proportional to the square root of the length.
Let T (1) and T (2) be the time period and L (1) and L (2) be the length of the pendulum respectively, accordingly;
T (1) : T (2) = √L(1):√L(2)
Or
L1:L2 = (Tl)2 : (T2)2
L1:L2 = (2)2 : (1)2
= 4:1