7,8a + 5,4a = 3,3
1,3x - 0,82x = 6
x - 0,28x = 36

7,8a+5,4a=3,3;

13,2a=3,3;

a=3,3:13,2;

a=0,25.

1,3x-0,82x=6;

0,48x=6;

x=6:0,48;

x=12,5.

x-0,28x=36;

0,72x=36;

x=36:0,72;

x=50