1.37. Решите уравнения: a) x + (x - 5) = 15; б) x + 3x = 20; в) x + (x + 7) = 31; г) 7x - x = 42;

a) $$x + (x - 5) = 15$$ $$x + x - 5 = 15$$ $$2x = 15 + 5$$ $$2x = 20$$ $$x = \frac{20}{2}$$ $$\mathbf{x = 10}$$ б) $$x + 3x = 20$$ $$4x = 20$$ $$x = \frac{20}{4}$$ $$\mathbf{x = 5}$$ в) $$x + (x + 7) = 31$$ $$x + x + 7 = 31$$ $$2x = 31 - 7$$ $$2x = 24$$ $$x = \frac{24}{2}$$ $$\mathbf{x = 12}$$ г) $$7x - x = 42$$ $$6x = 42$$ $$x = \frac{42}{6}$$ $$\mathbf{x = 7}$$