Simplifier chacune pour multiplier et avoir le même dénominateur

21/45 - 25/100 =
7/15 - 1/4 =
(7*4)/(15*4) - (1*15)/(4*15) =
28/60 - 15/60 = 13/60

21/45-25/100
= 21*100/45*100 -25*45/100*45
= 2100/4500 -1125/4500
= 975/4500
= 5*195/5*900
= 195/900
= 5*39/5*180
= 39/180
= 3*13/ 3*60
= 13/60