Answer:
The mass of the other body is 2.57 kg.
Step-by-step explanation:
The expression of the velocity of an object in an elastic collision is as follows;

Here,
,
are the masses of the objects,
,
are the initial speed of the objects and
is the final speed.
According to the given problem, a body of mass 3.6 kg makes an elastic collision with another body at rest and continues to move in the original direction but with 1/6 of its original speed.

Calculate the mass,
.

Put
,
and
.




Therefore, the mass of the other body is 2.57 kg.