Can anyone help me?

The diameter of a circle is 16. By what number must the radius be decreased in order to decrease the area of the circle by 48 pi?

I really am not sure what you are asking.....

The original area is   pi*64    if you want to decrease this by 48pi  then the new area must be 16pi units^2

So the radius would have to be 4 units, the diameter would be 8 units.

This is half of the orginal diameter. 

The current radius  is   diameter/ 2  = 16/ 2  =  8

And area  = pi * r^2

So....the current area is  pi (8)^2  = 64pi  units^2

Reducing by 48 pi units^2  means that he new area is  [ 64 pi - 48 pi ]  =  16 pi units^2

So......16 pi  = pi (r^2)       .....divide both sides by  pi ......  

16  = r^2  ⇒    4  = r

So....the current radius must be halved to reduce the area by 48 pi units^2