He has 20 pencils.
The ratio in the problem is 4 pens to 5 pencils meaning for every 4 pens there are 5 pencils.
So take this into mathematics 16pens/4pens = 4
5pencils * 4 is equal to 20 pencils
the ratio still works and it is still 4 to 5 so the answer is 20...pencils.
