Answer:
[tex]\huge\boxed{x=\dfrac{1}{3}}[/tex]
Step-by-step explanation:
[tex]\dfrac{1}{x}+\dfrac{1}{3x}=4\\\\\text{Domain:}\ x\neq0\\\\\text{find LCD}=3x\\\\\dfrac{1\cdot3}{x\cdot3}+\dfrac{1}{3x}=4\\\\\dfrac{3}{3x}+\dfrac{1}{3x}=4\\\\\dfrac{3+1}{3x}=4\\\\\dfrac{4}{3x}=\dfrac{4}{1}\qquad\text{cross multiply}\\\\(3x)(4)=(4)(1)\\\\12x=4\qquad\text{divide both sides by 12}\\\\\dfrac{12x}{12}=\dfrac{4}{12}\\\\x=\dfrac{4:4}{12:4}\\\\x=\dfrac{1}{3}\in D[/tex]
