SHM Motion Q: Find Block's Speed at Position(s)?

[maccha]

There's an example question from my textbook that I'm confused about:

A 500 g block on a spring is pulled a distance of 20 cm and released. The subsequent oscillations are measured to have a period of 0.80 s. At what position or positions is the block's speed 1.0 m/s?

The book solves this problem by using 20 cm as the amplitude. What I don't understand is, how do we know that this is the amplitude? I know it's stretched that far but couldn't the amplitude result in something different?

 

[flatmaster]

The spring is stretched 20 cm. Assuming energy is conserved, Your block will always oscilate back to the 20cm mark. The question assumes the idealized situation of no friction. Immagine the block on a cart or frictionless airtrack. If you were to stretch the spring to 20cm and give the cart some initial velocity, you would have some amplitude other than 20cm.