Answer:
The mass of the box, m = 31.25 kg
Step-by-step explanation:
It is given that,
Condition (1)
Force applied by Mary, F₁ = 79 N
Acceleration of the box,

Condition (2) :
Force applied by Mary, F₂ = 84 N
Acceleration of the box,

Let m is the mass of the box. It is assumed to find the mass of the box. The resultant forces acting on the box in condition (1) are :
...........(1)
f is the force of friction acting between the floor and the box.
The resultant forces acting on the box in condition (2) are :
..........(2)
On solving equation (1) and (2) we get :
f = 64.938 N
m = 31.25 kg
So, the mass of the box is 31.25 kg. Hence, this is the required solution.