X-1/2=3/4x; 1 4/5y=y+4; 3/4y-2/3=7/12y

1) х - 1/2 = 3/4x;

х - 3/4х = 1/2;

1/4х = 1/2;

х = 2.

2) 1 4/5y = y + 4;

((1 * 5 + 4)/5)у - у = 4;

4/5у = 4;

у = 5.

3) 3/4y - 2/3 = 7/12y;

3/4у - 7/12у = 2/3;

(3 * 3)/(4 * 3)у- 7/12у = 2/3;

9/12у - 7/12у = 2/3;

2/12у = 2/3;

у = 2/3 : 2/12 = 2/3 * 12/2 = (1 * 4/(1 * 1);

х = 4.