A tunnel, one-sided, with a 4m width is topped with two vertical walls of high diameter 2.5m.
A 2.5 m wide truck must pass.
what can be the maximum height of the truck?
what is a one-sided tunnel?
how do vertical walls have a diameter?
Maybe you could describe the shape of the tunnel cross-section. Heck, in doing so, you might actually see how to solve the problem...