a) x/2 = y/5 ; x + y = 21 b) x/y = 7/4 ; 2x - y = 20 c) x/2 = y/3 = z/4 ; x-y+z= -14

a) x/2 = y/5 ; x + y = 21

$\frac{x +y}{2+5 }$= $\frac{21}{7}$ = 3 

⇒  x/2 = 3 ⇔ x = 6 

⇒ y/5 = 3 ⇔ y = 15

b) x/y = 7/4 ; 2x - y = 20

$\frac{x}{7}$ = $\frac{y}{4}$ ⇒  $\frac{2x}{7.2}$ = $\frac{y}{4}$ = $\frac{2x - y}{14 - 4 }$ = $\frac{20}{10}$  = 2

$\frac{x}{7}$ = 2 ⇔ x= 2.7 = 14

$\frac{y}{4}$ = 2 ⇔ y = 2.4 = 8 

c) x/2 = y/3 = z/4 ; x-y+z= -14

$\frac{x}{2}$ = $\frac{y}{3}$ = $\frac{z}{4}$  => $\frac{x-y+z}{2-3+4}$ = $\frac{-14}{3}$ 

$\frac{x}{2}$ = $\frac{-14}{3}$ ⇒ x= $\frac{-28}{3}$ 

$\frac{y}{3}$ = $\frac{-14}{3}$ ⇒ y = -14

$\frac{z}{4}$  = $\frac{-14}{3}$ ⇒ z = -28 /2