(x - 20) * 3 + 40 = 340;
(x - 20) * 3 = 340 - 40;
(x - 20) * 3 = 300;
x - 20 = 300 : 3;
x - 20 = 100;
x = 100 + 20;
x = 120.
(x - 20) * 3 + 40 = 340;
(x - 20) * 3 = 340 - 40;
(x - 20) * 3 = 300;
x - 20 = 300 : 3;
x - 20 = 100;
x = 100 + 20;
x = 120.