WHIRLWIND EXPLANATION: Pokémon have a fixed, permanent score from 0 to 31 for each stat that affects how good that stat can ever get.

This calculator will figure out that score (called a "gene" or, more obtusely, an "IV") for you, so you can discard the unworthy. It's more accurate for higher-level Pokémon, so it's helpful to go into a level 100 wifi battle with someone and check your Pokémon's stats from there.

Stat calculator

Muk
Muk 		Base
stats		 Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Possible genes (IVs)
Level Level Level Level Level
HP 105
127–139
130–142
134–146
137–149
18
Attack 105
84–96
86–98
89–101
91–103
17
Defense 75
62–73
63–75
65–77
66–79
20
Special Attack 65
54–66
55–67
57–69
58–71
18
Special Defense 100
81–92
83–95
85–97
87–99
16–17
Speed 50
43–54
44–56
45–57
46–58
8–9

Hmm, I need more information to figure out your Pokémon's genes exactly. Try raising it to level 42 and entering its new stats.
Remember: if your Pokémon battles at all, you'll need to track the effort it gains. Consider saving your game and using Rare Candy instead.

And for your copy/pasting pleasure:

18 HP; 17/20 Physical; 18/16–17 Special; 8–9 Speed

or start over