To get this, lets begin by looking at the known.
In total, it costed $168. Since it costs $5 per tree, lets times:
5x8= 40
Now that we know that delivery fees were $40, lets subtracted that from the total.
168-40=128
That means our total now without delivery fees is $128.
Since we know that Yuri bought 8 trees, we can divide by that.
128/8=16.
Meaning each tree costs $16