A pool is filled by two pipes. It takes the first pipe 2 hours more and the second pipe 4 hours more to fill it than both pipes at the same time.
The book says that the first pipe fills the pool in 5 hours and the second one in 7.5 hours, so I think I did something wrong.
Anyway, here's what I did:

let z be the time it takes both pipes to fill the pool
let x be the speed of first pipe
let y be the speed of second pipe

So I get, that
\{ z(x+y)=x(z+2) \\ z(x+y)=y(x+4) \\ z(x+y)=1
Which solves for z=sqrt{8} x=4,8 and y= 6,8

I need some hints please.

I think you are right.

There should be a difference of 2 hours between the 2 values, from the first piece of data you gave, that is obvious.

This isn't all that rare in books like this, sometimes they edit a problem at the last moment and don't change the answer, that might have happened here.

Okie, thanks a lot Capuchin ;)

A pool is filled by two pipes. It takes the first pipe 2 hours more and the second pipe 4 hours more to fill it than both pipes at the same time.
The book says that the first pipe fills the pool in 5 hours and the second one in 7.5 hours, so I think I did something wrong.
Anyway, here's what I did:

let z be the time it takes both pipes to fill the pool
let x be the speed of first pipe
let y be the speed of second pipe

So I get, that
\{ z(x+y)=x(z+2) \\ z(x+y)=y(x+4) \\ z(x+y)=1
Which solves for z=sqrt{8} x=4,8 and y= 6,8

I need some hints please.
divide the answer by which you think you had first or add up all the numbers and divide the by each number on at a time and see if you get the right answer