Bjr
21/72 = 7/(72/3) = 7/24 = 35/(5*24)=35/120
a) 21/72 = 7/24=35/120
5/7 = (5*9)/63 = 45/63 = 45/(7*9)=45/63
b) 5/7 = 45/63 =45/63