Momentum will decrease by 20%.
The equation for an object's kinetic energy looks like this
The equation that describes the momentum of an object is
So, you know that the object's kinetic speed decreases by 36%. This means that it now stands at 64% of its initial value.
Since the mass of the object is assumed to be constant, this decrease in kinetic energy correlates with a decrease in velocity.
Take the square root from both sides of the equation to get
Now use this relationship between the two velocities of the object to express the initial and final momentum
This means that the momentum decreased by
Expressed in percentages, this is equivalent to