why is my battery draining after 40%?

Hi. I've charged my Samsung A5 2016 full, and used it as usual my activity. After my battery percentage left 40%, suddenly my smartphone was off directly. It happened almost 6 days a go until now.
And I try to charge my smartphone only until 43%, and it's still 43% until 5hours in normal activity (open social media, editing picture). After that the battery reduce based on the way I used it, not drop like if I was full charged.

Does anyone could give me some advice about that? Thanks a lot

Welcome to Android Central! Try recalibrating the battery sensor software by charging up to 100%, then using it normally until it shuts off automatically from low battery, then charge back up to 100%. If the problem persists, then the battery might be defective or failing -- contact Samsung if it's still under warranty, or bring it to a repair shop.