Answer
length=12 cm
Work Step by Step
In a rectangle diagonal make two congruent right triangles.
So by Pythagoras theorem
$Diagonal^{2}= width^{2}+length^{2}$
$20^{2}=16^{2}+length^{2}$
$400=256+length^{2}$
$144=length^{2}$
length=12 cm
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