Explanation:
If Omar has $350 to spend on the garden he is designing, then he has possibility to buy 11 triangle stones and 26 border pieces, if the construction look like below, because 20×11 + 5×26 = 350.20×11+5×26=350.
So, the length of the path is 11×3 = 33 units, if the length of a side of such triangle is 1 unit.