Step-by-step explanation:
Let us assume his monthly salary as x. According to the question,
➝ Money spent on rent + Money spent for utility bill + Remaining money = His salary
[tex]\longrightarrow\sf {\dfrac{1}{3}x + \dfrac{1}{7}x + 759 = x} \\ [/tex]
[tex]\longrightarrow\sf {\dfrac{7x + 3x + 15939}{21}= x} \\ [/tex]
[tex]\longrightarrow\sf {\dfrac{10x+ 15939}{21}= x} \\ [/tex]
[tex]\longrightarrow\sf {10x+ 15939= 21x} \\ [/tex]
[tex]\longrightarrow\sf {15939= 21x - 10x} \\ [/tex]
[tex]\longrightarrow\sf {15939= 11x} \\ [/tex]
[tex]\longrightarrow\sf {\cancel{\dfrac{15939}{11}}= x} \\ [/tex]
[tex]\longrightarrow\underline{\boxed{\bf {1449= x}}} \\ [/tex]
Therefore, his monthly income is $1449.