<span>а)3/4=15/20 = 3*5/4*5 - верно</span>
<span> б)7:5=77/55 = 7*11/5*11 - верно</span>
<span> в)12/18=14:21-верно</span>
<span> 2*6/3*6=2/3</span>
<span> 14:21=2*7:3/7=2/3</span>