When you add up (20N down) and (10N up),
you get a sum of (10N down).
The mass of the object has no effect on the forces.
Now ... I see 5 points for the answer.
Where are the other 25 coming from ?
Answer:
Net Force = 10 N
acceleration: 
Explanation:
1) The net force on the box is the applied force (F = 13 N) minus the friction force (f = 3 N), Therefore, net force = 10 N
See attached free body diagram.
2) To find the box's acceleration, we need first to find the mass of the box (since they are giving us its weight in Newtons). To do such we use the equation for weight and solve for the box's mass:

Now we can find the box's acceleration by using the equation for the net force:

Answer:
22m/s
Explanation:
Mass, m=60 kg
Force constant, k=1300N/m
Restoring force, Fx=6500 N
Average friction force, f=50 N
Length of barrel, l=5m
y=2.5 m
Initial velocity, u=0

Substitute the values

m
Work done due to friction force

We have 
Substitute the values


Initial kinetic energy, Ki=0
Initial gravitational energy,
\
Initial elastic potential energy


Final elastic energy,
Final kinetic energy, 
Final gravitational energy, 
Final gravitational energy, 
Using work-energy theorem

Substitute the values






Answer:
5 m/s²
Explanation:
Given:
v₀ = 0 m/s
v = 40 m/s
t = 8 s
Find: a
a = (v − v₀) / t
a = (40 m/s − 0 m/s) / 8 s
a = 5 m/s²
We're millions of miles away from mars