Приведите дроби: 4/9, 5/6, 7/2 к знаменателю 18
7/8, 5/16, 21/40 к знаменателю 80

4/9 = 4*2/9*2 = 8/18
5/6 = 5*3/6*3 = 15/18
7/2 = 7*9/2*9 = 63/18

7/8 = 7*10/8*10 = 70/80
5/16 = 5*5/16*5 = 25/80
21/40 = 21*2/40*2 = 42/80

18=9•2=6•3=2•9
поэтому
4/9=(4•2)/(9•2)=8/18
5/6=(5•3)/(6•3)=15/18
7/2=(7•9)/(2•9)=63/18

80=8•10=5•16=2•40
поэтому
7/8=(7•10)/(8•10)=70/80
5/16=(5•5)/(5•16)=25/80
21/40=(21•2)/(40•2)=42/80