"3,9/14 : 2,1/7 = x : 1,5"

x = (3,9/14) * (1,5) / (2,1/7)

x = (3,9 * 1,5) / (14 * (2 + 1/7))

x = 5,85 / (14 * (15/7))

x = 5,85 / (210/7)

x ≈ 5,85 * (7/210)

x ≈ 0,195