Answer
The captain thought that they were 993 meters above their correct altitude when he started the rapid descent.
Work Step by Step
When the plane started the rapid descent, the plane was at an altitude of 1450 meters. The captain thought they should descend to an altitude of 1500 feet.
We can convert 1500 feet to units of meters:
$1500~ft \times \frac{1~m}{3.28~ft} = 457~m$
We can find the difference between 1450 meters and 457 meters:
$1450~m-457~m = 993~m$
The captain thought that they were 993 meters above their correct altitude when he started the rapid descent.
