The character is at either 1 or 2 failed death saves.
So our character has 20 hit points, and is damaged for 25 damage. This reduces their hit points to 0; we see in the rules for Hit Points:
A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.
Whenever a creature takes damage, that damage is subtracted from its hit points.
Having reached 0 hit points, they are now unconscious:
When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.
They do not die outright, because the rule for Instant Death states:
When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
They were reduced to 0 hit points and there was only 5 damage remaining, so they do not die instantly.
Next they are hit for 15 damage, and the rules for damage at 0 hit points say:
If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.
If the 15 damage was an attack from more than 5 feet away, or damage from an effect that isn't an attack (like a spell that forces a dexterity save), you suffer one failed death save. If it was an attack from within 5 feet, you suffer two, since critical hits (don't forget to roll those dice twice) cause two failed saves and the rules for the unconscious condition state:
Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.
You do not die instantly because the damage did not exceed your hit points maximum. There is nothing in these rules that would indicate that damage from multiple attacks can accumulate and cause instant death, instead, those attacks causing failed death saves.