Two or more waves combining to produce a wave with a smaller displacement is called

Destructive interference

Explanation:
Interference is a term used to refer to the combination of two waves or their overlap
There are two types of interference:
1- Constructive interference: This happens when the waves overlap/combine to produce a resultant wave with a larger displacement
2- Destructive interference: This happens when the waves overlap/combine to produce a resultant wave with a smaller displacement

Hope this helps :)

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.