Let AC represent the initial height and point A represent the initial position of the plane.
Let point B represent the position where plane lands.
Angle of depression = ∠EAB = 20°
Now, seg AE || seg BC
∴ ∠ABC = ∠EAB … [Alternate angles]
∴ ∠ABC = 20°
Speed of the plane = 200 km/hr
= 200 × 1000/3600 m/sec
= 500/9 m/sec
∴ Distance travelled in 54 sec = speed × time
= 500/9 × 54 = 3000 m
∴ AB = 3000 m
In right angled ∆ABC,
sin 20° = AC/AB ….[By definition]
∴ 0.342 = AC/3000
∴ AC = 0.342 × 3000 = 1026 m
∴ The plane was at a height of 1026 m when it started landing.