magic the gathering how to play fireball

[Levi]

how do you play fireball in magic?

[Twanos]

you tap R + X fire ball then deals X damage to target creature or player. You may pay 1 per extra creature you want to target but then X must be divided evenly.

so R+10 will hit 1 guy for 10
R +1 +10 will hit 2 guys for 5
R + 2 + 10 will hit 3 guys for 3. Since you can't do 3.33 damage to each and the card says you must round down so 3 to each.