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
