Question

25 15
---- x ----
30 20

18 8
--- x ---
24 21

15 16
--- x ---
4 9

16 12
--- x ---
28 8

Answer 1

Bueno: Multiplicamos los términos de arriba y de abajo. Y luego se simplifica 

$\frac{25}{30} . \frac{15}{20}$ = $\frac{375}{600}$  = $\frac{5}{8}$ 

$\frac{18}{24} . \frac{8}{21}$ = $\frac{144}{504}$ = $\frac{12}{7}$ 

$\frac{15}{4} . \frac{16}{9}$ = $\frac{240}{36}$ = $\frac{20}{3}$ 

$\frac{16}{28} . \frac{12}{8}$ = $\frac{192}{224}$ = $\frac{6}{7}$ 

Espero haberte ayudado!!