Te basic formula is: X · 25% = Z
In your case: X · 25% = 6,000
You get X alone: X = 6,000 / 25%
And operate: X = 24,000
If you think it, you can make it even faster. 25% is a fourth. So, in order to get the full number, you have to multiply by 4: 6x4=24 -> 24,000.