If x +1% = 101x/100 then +1% = *101/100
You have to iteratevely multiply when adding bonuses. Alike applying a single bonus.
A Percentage is a Ratio it IS strictly relative.
You cannot add two different things. Nonetheless you can Multiply them.
x + 1% + 1% = 102x/100 10201x/10000
We cannot play with science.
We cannot "do" mathematics the way we like!
Riot chooses whether they want stats to stack additively or multiplicatively, depending on how they want the stat to work.
For CDR, it's additive. You have 10% CDR and buy another 10%, you will then have 20% CDR.
For tenacity, it's multiplicative. If you have 10% tenacity (Irelia in a 1v1) and buy Merc Treads with 30% tenacity, a 1-second stun would actually last 1*0.9*0.7 = 0.63 seconds, which is a total of 37% CC reduction.
Stacking effects, such as 
's innate passive, or the 
's W passive should be interpreted mathematically as [effect per stack]x[number of stacks]. Hence, at the max of 5 stacks, Kayle is reducing armor and mr by 15%, and likewise, Volibear is getting +60% Attack Speed (rank 5 W). Now if you want to compute total % armor reduction from 
's innate passive plus a fully stacked 
unique passive. You would be looking at enemy armor equal to 100%-15%=85%, 100%-30%=70%, and hence % of total armor remaining is 0.85*0.7=0.595=59.5%. Hence they are losing 100%-59.5%=40.5% of their armor.
