Lets say i cast weakness to magic 100% and weakness to fire 100% on a monster, after which i blast the monster with a 30 points fire damage spell.

Would the total damage be 30*300% = 90 or 30*200% = 60 (for the first 100% weakness) and then 60*200% = 120 (for the second 100% weakness?)

Also, from the wiki: When casting a spell which includes a Weakness to Magic effect on a target already affected by Weakness to Magic, the original Weakness to Magic effect will affect all effects in the spell up to and including the Weakness to Magic effect.

which means casting two 100% weakness to magic spells on a target would give the target 200% weakness to magic?