Please help with the 4th one

The whole field is square, and its total area is 25 acres. 
It's divided up into 25 little sections. 
Each little section is a square, and its area is 1 acre. 

So the whole thing looks like a square tray with 25 little square blocks in it.
The blocks are in 5 rows, and there are 5 blocks in each row.

There are 16 blocks around the edge of the square tray, and 9 in the middle
that don't touch the edge.

Every block costs $1,200 to start out.  But the ones around the edge are
more expensive.  Anything on the edge costs an extra $150 for each side
of the little block that touches the edge.

Most of the 16 little blocks around the outside have only 1 side that touches
the edge.  But the ones in the corners of the tray have 2 sides along the edge.

You may want to have a look at the picture I attached.

That should be plenty to put you in a position to work out the costs.