mysql data types: define as char but work as int
If there is no specific requirement for you to store the data in that manner then you SHOULD change the datatype of the column and make it to INT or Decimal as per your requirement.( This would be a suggestible approach as you will need to do a lot of calculations and there is no inbuilt function to calculate 'K' values)

However if there is not way out, you can use a replace function to replace the 'K' with a space or '000' and use INT/DECIMAL function to  convert the same for your  calculations.  Syntax would be something as below 

SELECT * FROM TAX_TABLE WHERE CAST(REPLACE(TAX,'K','') AS DECIMAL) &gt; 15 


OR

SELECT * FROM TAX_TABLE WHERE CAST(REPLACE(TAX,'K','000') AS DECIMAL) &gt; 15000