The Athlete Feat has a point that says:

climbing doesn't halve your speed.

The Centaur Equine Build trait says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

This question has nothing to do with the difference between listed climb speed and walking speed.

The Athlete feat (PHB, p. 165) has a point that says:

climbing doesn't halve your speed.

The Centaur's Equine Build trait (GGtR, p. 16) says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

This question has nothing to do with the difference between listed climb speed and walking speed.

The Athlete Feat has a point that says:

climbing doesn't halve your speed.

The Centaur Equine Build trait says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

The Athlete Feat has a point that says:

climbing doesn't halve your speed.

The Centaur Equine Build trait says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

This question has nothing to do with the difference between listed climb speed and walking speed.

The Athlete Feat has a point that says; climbing doesn't halve your speed. The Centaur Equine Build trait says; any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.The normal climb normally costs half you speed and athlete negates that but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?

The Athlete Feat has a point that says:

climbing doesn't halve your speed.

The Centaur Equine Build trait says:

any climb that uses both hands and feet is especially difficult for you because of your equine legs. When you make such a climb, each foot of movement costs you 4 extra feet, instead of the normal 1 extra foot.

The normal climb normally costs half your speed and athlete negates that, but the equine build costs you 80% of your speed instead of 50%. How would these two interact with each other?