[Math4u] got some math I need help with

I’m a Mature college student that is not getting the math

 

Here are 3 ? I have answers but need the formulas

 

#1: A fuel storage depot had an 8-weeks supply on hand. However, cold weather caused the supply to be used in 6 weeks when 20000l extra were used each week. How many liters were in the original supply.    (answer: 480 000)

 

So X= the total supply       

 

 

#2 : A 10 l cooling system contains a solution of 20% antifreeze and 80% water. How much solution must be replaced with pure Antifreeze to raise the strength to 50% antifreeze.   (answer is 3.75L)

 

 

#3 cos(3pi/5)    = -.309017

 

 

Any help or direction would be great

 

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