In such questions we mustuse examples.....
Let's imagine that the shortest size is 1
So as it is half of the second the second must be2
As it is the third of the longest size it must be 3
To find the perimeter we must add the three sides.
1+2+3=6
Therefore the perimeter is the double of the longest size
or
The longest size is half of the perimeter
We must take one or two more examples to be sure...
I will take another one
Let's imagine that the shortest size is 5
So as it is half of the second the second must be10
As it is the third of the longest size it must be 15
To find the perimeter we must add the three sides.
5+10+15=30
Therefore the perimeter is the double of the longest size
or
The longest size is half of the perimeter
I wish I helped you ;)