This is actually a quote of a quote nothing wrong with asking a question, more power for you for having done it, but also feel free to do a search for answers to questions like this, you might just find an answer
Slowing effects follow particular rules when stacking:
If a champion is affected by multiple slows, the strongest one will be fully applied, while the others are applied sequentially with 65% reduced effectiveness down to 35% of its original strength.
Slow effects derived from items' passives do NOT stack, even when coming from different champions. Only the strongest item slow is applied at any moment. The exception to this is the active effect of Randuin's Omen, Hextech Gunblade and Bilgewater Cutlass, which stacks with other slowing items normally because they are considered champion abilities for slow stacking purposes.
Attacking a champion with the Blessing of the Lizard Elder buff when the target is already affected by that slow will reapply the debuff, resetting its duration. It will NOT increase the slow amount. The same holds true for the Exhaust summoner spell.
A champion with a raw movement speed of 400 is affected by a 2 second long 40% slow and a 5 second long 20% slow at the same time.
For the first two seconds, he will have (1 − 0.4) × (1 − 0.2 × 0.35) = 0.6 × 0.93 = 55.8% of his original movement speed (a 44.2% effective slow). So he will have 400 × 0.558 = 223.2 raw movement speed.
For the following three seconds, the effective slow will be 20% and so he will have 400 × 0.8 = 320 movement speed.
After that, he will return to his normal speed of 400.
Source: Slow page on LoL Wikia