how to decrease column values by 20 percent using mysql
You need the reciprocal value of 20%, because you're new value isn't 120% it is the new 100% value.

round(listprice * 1.2,2) -- increase by 20%


The reciprocal for above +20% would be:

round(listprice / 1.2,2) -- decrease by 17.777777% 


Or put basically: If you want to counteract an increase of 20% then you need to reduce by 17.7777777%. 

You can verify this on Windows with the calculator. Beware of the round() function!



Mathematics Explanation

Multiplying a value with a number is another way of adding. If you want to add 100% to a given value, then you can multiply the value with 2:

listprice * 2 


If we replace listprice with 100 then we have:

100 * 2 = 200


This is an increase of 100% compared to the starting value of 100. 

If you want to increase a value by 20% then you want to add a fifth of the base value. This is the equivalent of multiplying by 1.2:

100 * 1.2 = 120


Now multiplying by 1.2 can counteracted by dividing the result by 1.2 again:

120 / 1.2 = 100


This is the equivalent of what you are looking for. You want to counteract the addition of listprice +(listprice *(20/100)). 



Working Example

A working example can be found at db&lt;>fiddle (MySQL 8.0)



Using the Original Addition

After poring over your equation last night and this morning I found a way of using your original "addition".

                20   
x = y + ( y * ----- )
               100   


This can be converted to:

                20   
x = y * ( 1 + ----- )
               100   


Which as you may note is a multiplication again. So if you were to want the original value of y, then all you need to do is to divide by original multiplicand:

                20   
y = x / ( 1 + ----- )
               100   


Or written as :  

          x
     ------------
y =         20        
     (1 + ----- )  
           100


In your MySQL equation this would be:

listprice = round(listprice / (1 + (20/100)),2)




Working Example Using Addition

A working example can be found at db&lt;>fiddle (MySQL 8.0)