At 70% off, the rope length is x/3, x/3-y = 4................①.

At 40% off, the rope length is x/4, x/4-y =- 1...........②.

X/3-X/4 of ①-② = 5.

What about up there? Answer b is right.

In fact, I think

At the 30% discount, there are three strands of rope, x/3 for each length, 4 feet more and 4 feet more. x/3-y=4×3................①

At a 40% discount, there are four strands of rope, each strand is x/4, less 1 foot, and each strand is less 1 foot. x/4-y=- 1×4...........②

I don't want your points, I just want to solve the problem.