Measurement of force (statics) body 1 and 2 are pulling the balance by 20 N
what does the balance read?
same question but without 2.
for 1st question I thought the balance will read zero force because they are two opposite forces of the same magnitude but it's wrong. the correct answer is 20N I don't know why

 A: The net force on the spring balance is zero. This means that the balance as a whole will not accelerate since ($F_{net} = ma$).

for 1st question I thought the balance will read zero force because they are two opposite forces of the same magnitude but it's wrong. the correct answer is 20N

Hmm.. let's take a look at the mechanism of a spring balance

You can see that only one end of the balance can move the spring and the pointer at the same time.
Assuming the load portion of the balance towards 2 (you can also take the other way) :
So when 1 and 2 both were trying to pull it by $20N$, the person 1 was only stopping the balance from accelerating in the direction of 2.  But only because 1 was at the other end, the force of 2  ($20N$) was able to stretch the spring in the balance (without accelerating) and thus it should show the $20N$ mark.  If 1 leaves, the balance will accelerate towards 2 without showing any results.
You can try this experiment at home.  So you should know that if the other end of the balance is left free, then the balance will not measure anything. You need someone or something to hold the other end of the balance and this is what happens when you attach a balance with a free-falling body. The pointer shows zero deflection.
Hope it helps ☺️.
A: The net force on the spring balance is zero, but that is not what the spring balance is measuring. You could replace either person by a fixed wall. The remaining person is then exerting a force of $20$ N on a spring balance that is fixed at the other end, and so the spring balance will read $20$ N.
A: For every weighing scale the force you exert is always counterbalanced by an equal and opposite force - that's the third Newton's law. If the scale lies on the ground, your weight would be opposed by the normal force coming from the ground, which is fixed; the same would happen with a Newton meter (or a spring balance), hanging down hinged on a wall. But if you suspend the spring meter and pull it toward yourself, it will consequently accelerate with your hands. You need an equal strength from the opposite side to fix the balance and let it measure the force, which is in this case 20 N.
