Answer :
Given:
[tex]\frac{3}{10}+\frac{5}{10}=\frac{4}{x}[/tex]Solve the equation,
[tex]\begin{gathered} \frac{3}{10}+\frac{5}{10}=\frac{4}{x} \\ \text{Find the least common multiple of 10 ,x=10x} \\ \text{ multiplied by 10x} \\ 10x(\frac{3}{10}+\frac{5}{10})=10x\cdot\frac{4}{x} \\ 10x\cdot\frac{3}{10}+10x\cdot\frac{5}{10}=10\cdot4 \\ 3x+5x=40 \\ 8x=40 \\ \text{Divide by 8} \\ \frac{8x}{8}=\frac{40}{8} \\ x=5 \end{gathered}[/tex]Answer: x=5