What is the answer to this problem?

Hey there!

To start, an isosceles triangle is any triangle with two sides of the same length. 

In this case, you are given two measurements: 20 and 30.

This means that the third side of the triangle can either be 20 or 30, since two of the sides must equal one another.

Because the question is asking for the greatest possible perimeter of the triangle, it would only make sense that the third side would have to be 30 units in length to make the greatest possible perimeter since it is the possible length that is greatest in value (20 < 30).

Therefore, knowing that the sides of the triangle are now 20, 30, and 30, add the lengths up to find the perimeter:
20+30+30
=80

Therefore, the largest possible value of the perimeter would be 80 units.

Hope this helps and I hope you have a marvelous day! :)