a) x • 18/13 = 36/39

x = 36/39 : 18/13 = 36/39 • 13/18 = 2/3

б) х • 15/12 = 30/36

х = 30/36 : 15/12 = 30/36 • 12/15 = 2/3

в) 1,8/х = 0,9/10

х = (1,8 • 10)/9 = 2

г) 27/18 = х/3,6

х = (27 • 3,6)/18 = 5,4