Find wavelength for a standing wave

[jwxie]

Homework Statement

Two sinusoidal waves with identical wavelengths and amplitudes travel in opposite directions along a string with a speed of 10 cm/s. If the time interval between instants when the string is flat is 0.50 s, what is the wavelength of the waves?

Homework Equations

v = w/k = lamda * frequency

The Attempt at a Solution

The solution gives T/2 = 0.50 and T = 1 s
Why do we have to multiple by two? In fact, if I didn't take twice the given time, i would have this
frequency = 1/T = 0.5 still which doesn't make sense...

How does "time interval between instants" implies twices? I know between is used when comparing two things. But I just thought that 0.5 was the time the string stayed flat.
Thank you very much.

 

[ehild]

A sine function takes the zero value twice during a period T. (sin(0)=sin(pi)=0) If the time interval between successive zero values is 0.50 s the whole time period T is twice as long: 1.00 s.

ehild