If you want to find the difference of 6/25 - 4/5 what common denominator should you choose? why?

You should always choose the LCM is a common denominator

LCM (5,25) = 25

the first fraction already has 25 as its denominator

the second fraction we must multiply by 5 above and below

so

6/25-4*5/5*5

6/25-20/25 =

-14/25