Why does Kass go through 3 MR items like a knife through butter without pen

That 135 MR. + Evelynn's...lets call it level 15 base MR (49) adds up to 184. 184/284=65% reduction. What items did Kassadin have (his masteries are probably relevant too, if you can get them)

That's 120 MR, plus your base, which is probably around 35-40, so let's assume 160? That's 61.538% DR against AP. Locket has a decent portion of health, but unless you have the health to back up that much MR, then you really weren't building into anything useful. If your other items were a RoA and a Rylais, I could see you being extremely tanky vs Kass, but since you were playing Eve, I don't think you were.

Kass probably just had enough AP to invalidate your MR. Can't know for sure without a full build from both of you, though.

well this was easy

He didn't have to penetrate much magic resist. You say you were playing eve and you had these items.. but if you have no hp for your items to defend then he doesn't have to do much, it scales multiplicatively.

60 MR 0 hp 2750g 20 mr 400 hp 2500g 40 mr 0 hp 720g

HP*(1+MR/100) = effective hp against magic damage

keep in mind that most mages run something like.. what was it 10 magic penetration, 11 maybe? That's from runes then more from masteries. Lets say its 15 then there's another 15 from penetration boots so that comes out to about 30 MR penetration so their natural build path is going to partly counter your purchases up to a point as well. But then midlaners also tend to level fast and snowball.. but back to the math.

with this build you add a total of 400*(1+120/100) = 880 effective hp for a total of 5970g

880 effective hp for 5970g is really low. You can can get a + for 3200g and have 1007.5 effective hp against magic

would have been 1280 effective hp for 2750 + 2850 = 5600g

Building was a mistake basically if kass was shredding you.. why would even get locket anyways?

21/1/8

Let's not forget that. It doesn't matter if you have some MR if he has so much AP he still melts you anyways.