7. 5 3/5 : 3 1/2 = 5 1/4 : x

\(\dfrac{28/5}{7/2}=\dfrac{21/4}{x}\).

\(x=\dfrac{7}{2}\cdot\dfrac{21}{4}\cdot\dfrac{5}{28}=\dfrac{105}{32}=3\dfrac{9}{32}\).

Ответ: \(x=3\dfrac{9}{32}\).