Hello :) I hope this solution helps you. When you encounter this kind of problems, it will be good if you are able to come up with an equation.

In this case, the equation is :
1 string + 3 ropes = 5 ropes

I hope that you are able to see that from the above equation, 1 string = 2 ropes.

With that, you will be able to find the length of 1 rope and proceed to solve the question with ease.

I hope this explanation helps you :) Do feel free to contact me if you have any other questions!