25=5*5

20=2*2*5

you want the least common denominator or the least common multiple(LCM)

to be a multiple of 25, the LCM must contain all of 25's factors (the two 5's)

so far we have LCM=5*5, but we are not done yet

the LCM must be a multiple of 20 also, so the LCM must contain all of 20's factors

the LCM must contain two 2's and a 5(20's factors)

*But* we only use the factors from 20 that we need, not the ones that we already have

we already have the 5 from 25, so we don't use the 5 from 20

however, we don't have the two 2's from 20

so we put in the two 2's as factors in the LCM: LCM=5*5*2*2=100

by the way, how do we know this is the LCM ?(*least* common multiple)

if we divide 100 by 20 we get 5 and if we divide 100 by 25 we get 4; 5 and 4 have
*no* common factors except the number 1

11/25=44/100 and 9/20=45/100

45/100>44/100, so 9/20>11/25

by the way, if you ever get stuck for a common denominator you can always multiply the two denominators together but you will not necessarily get the LCM(you will get the LCM this way if the two numbers have no common factors !)

so if you multiply 25 and 20 you get 500

but now you are working with much larger numbers and therefore there is more chance of making an arithmetic mistake, so the best thing to do is find the LCM,
*but* if you get stuck you can do this and at least solve the problem !!