28 a)
[tex] \frac{8}{12} = \frac{3}{x} \\ x= \frac{12*3}{8} = \frac{36}{8} =4,5[/tex]
29 a)
[tex] \frac{12}{5} = \frac{x}{20} \\ x= \frac{12*20}{5} = \frac{240}{5} =48[/tex]
36 a)
[tex] \frac{x}{a} =5[/tex] ⇒x=5a
[tex] \frac{y}{b} =5[/tex] ⇒y=5b
[tex] \frac{z}{c} =5[/tex] ⇒z=c5
[tex] \frac{x+y+z}{a+b+c} = \frac{5a+5b+5c}{a+b+c} = \frac{5(a+b+c)}{a+b+c} =5[/tex]
