How much tension is there in a rope that hold a 20-N bag of apples at rest?

Since the bag is not accelerating vertically, we know that all of the vertical forces

on it are balanced, that is, they add up to zero.

There is a downward force of 20-N on the bag, because of gravity.

The upward forces on it must also add up to 20-N.

The only upward force on the bag is the tension in the rope.

So the tension must be 20-N.