The force of gravity on the 1 kg box is 2 N.
The formula for the force of gravity F on an object with mass m is
F = (GmM)/d²
Since G, M , and d are the same for both objects, (GM)/d² = a constant = k ). Then, we can write
F = km
F₂/F₁ = m₂/m₁
F₂ = F₁ × m₂/m₁
If the 2.5 kg box is object 1,
F₂= 6.1 N × (1 kg/2.5 kg) = 2 N
Note: The answer can have only one significant figure because that is all you gave for the mass of the 1 kg object.