There must be movement in the same direction as the force put on the object. Hope this helps!
Answer:


Explanation:
Wave 1, 
Wave 2, 
Wave 3, 
Wave 4, 
The general equation of travelling wave is given by :

The value of
will remain the same if we take phase difference into account.
For first wave,



For second wave,



For the third wave,



For the fourth wave,



It is clear from above calculations that waves 1 and 3 have same time period. Also, wave 2 and 4 have same time period. Hence, this is the required solution.
By the use of a Accumulators
90t - .100 = 75t
90t - 75t = 0.100
15t = 0.100
t = 0.0067
<u>t = 0.0067 hours or 24.12 seconds</u> (.0067 * 60 minutes * 60 seconds)