Đáp án:
a. $\frac{x}{16}$ = y ; x + y = 68
<=> x = 16y ; 16y + y = 68
<=> x = 16y ; y = $\frac{68}{17}$ = 4
<=> x = 64 ; y = 4
b. $\frac{x}{3}$ = $\frac{y}{4}$ ; x + y = 28
<=> x = $\frac{3y}{4}$ ; $\frac{3y}{4}$ + y = 28
<=> x = $\frac{3y}{4}$ ; $\frac{7y}{4}$ = 28
<=> x = $\frac{3y}{4}$ ; 7y = 112
<=> x = $\frac{3y}{4}$ ; y = 16
<=> x = 12 ; y = 16
c. $\frac{x}{-2}$ = $\frac{y}{7}$ ; x + y = 105
<=> x = $\frac{-2y}{7}$ ; $\frac{-2y}{7}$ + y = 105
<=> x = $\frac{-2y}{7}$ ; $\frac{5y}{7}$ = 105
<=> x = $\frac{-2y}{7}$ ; 5y = 735
<=> x = $\frac{-2y}{7}$ ; y = 147
<=> x = -42 ; y = 147
d. $\frac{x}{-12}$ = $\frac{y}{-17}$ ; x - y = 8
<=> x = $\frac{12y}{17}$ ; $\frac{12y}{17}$ -y = 8
<=> x =$\frac{12y}{17}$ ; $\frac{-5y}{17}$ = 8
<=> x = $\frac{12y}{17}$ ; -5y = 136
<=> x = $\frac{12y}{17}$ ; y = $\frac{-136}{5}$
<=> x = $\frac{-96}{5}$ ; y = $\frac{-136y}{5}$
