Gotta solve 'em all! This is a classic system of equations disguised as a Pokémon puzzle. Looking at Xerneas, Yveltal, and Zygarde (the Legendary trio from Pokémon X/Y), we've got ourselves a simultaneous equation problem that would make Professor Oak proud.
From the first equation: X + Y + Z = 100
From the second: 15X + 9Y + Z = 300
Solving this system (and sparing you the algebraic gymnastics), we get X = 5, Y = 10, and Z = 45. The Kalos legendaries have numerical values that perfectly balance, just like they maintain balance in the Pokémon universe!
Next time someone says math isn't useful in the real world, remind them how else would we determine the numerical value of fictional pocket monsters?