Answer
Find out the ratio of sharpened pencils to unsharpened pencils .
To proof
As given
In a box of 36 pencils, 20 are sharpened
Number of unsharpened pencil = 36 -20
= 16
Now find out the ratio of sharpened pencils to unsharpened pencils .

simplify

Therefore the ratio of sharpened pencils to unsharpened pencils is 5 :4 .
Hence proved